diff --git a/src/const.h b/src/const.h
index 6710fe9e52dfdbede468282d84d4441b923a59fe..49b8a165e127863adc2739f6681e36d700ab8ac4 100644
--- a/src/const.h
+++ b/src/const.h
@@ -60,7 +60,8 @@
/* SPH variant to use */
//#define MINIMAL_SPH
-#define GADGET2_SPH
+//#define GADGET2_SPH
+#define HOPKINS_PE_SPH
//#define DEFAULT_SPH
/* Self gravity stuff. */
diff --git a/src/debug.c b/src/debug.c
index 487fd53e74399ef7bd1802704adbf84bbc3dc0a3..9e14ad27e09e083a1b60a49e1eb2009a1761bdc7 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -41,6 +41,8 @@
#include "./hydro/Minimal/hydro_debug.h"
#elif defined(GADGET2_SPH)
#include "./hydro/Gadget2/hydro_debug.h"
+#elif defined(HOPKINS_PE_SPH)
+#include "./hydro/PressureEntropy/hydro_debug.h"
#elif defined(DEFAULT_SPH)
#include "./hydro/Default/hydro_debug.h"
#else
diff --git a/src/hydro.h b/src/hydro.h
index b2ae9d57c399ecea818e9f3dc7db238e01487a9a..30066662252b0c1e6dc770915f7d64e7a084e015 100644
--- a/src/hydro.h
+++ b/src/hydro.h
@@ -34,6 +34,10 @@
#include "./hydro/Gadget2/hydro.h"
#include "./hydro/Gadget2/hydro_iact.h"
#define SPH_IMPLEMENTATION "Gadget-2 version of SPH (Springel 2005)"
+#elif defined(HOPKINS_PE_SPH)
+#include "./hydro/PressureEntropy/hydro.h"
+#include "./hydro/PressureEntropy/hydro_iact.h"
+#define SPH_IMPLEMENTATION "Pressure-Entropy formulation of SPH (Hopkins 2013)"
#elif defined(DEFAULT_SPH)
#include "./hydro/Default/hydro.h"
#include "./hydro/Default/hydro_iact.h"
diff --git a/src/hydro/PressureEntropy/hydro.h b/src/hydro/PressureEntropy/hydro.h
new file mode 100644
index 0000000000000000000000000000000000000000..e223aa4d245a6766c75b5309ceeb4d54bea1de42
--- /dev/null
+++ b/src/hydro/PressureEntropy/hydro.h
@@ -0,0 +1,377 @@
+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_PRESSURE_ENTROPY_HYDRO_H
+#define SWIFT_PRESSURE_ENTROPY_HYDRO_H
+
+/**
+ * @file PressureEntropy/hydro_part.h
+ * @brief Pressure-Entropy implementation of SPH (Particle definition)
+ *
+ * The thermal variable is the entropy (S) and the entropy is smoothed over
+ * contact discontinuities to prevent spurious surface tension.
+ *
+ * Follows Hopkins, P., MNRAS, 2013, Volume 428, Issue 4, pp. 2840-2856
+ */
+
+#include "adiabatic_index.h"
+#include "dimension.h"
+#include "equation_of_state.h"
+#include "hydro_properties.h"
+#include "kernel_hydro.h"
+
+/**
+ * @brief Returns the internal energy of a particle
+ *
+ * @param p The particle of interest
+ * @param dt Time since the last kick
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_internal_energy(
+ const struct part *restrict p, float dt) {
+
+ const float entropy = p->entropy + p->entropy_dt * dt;
+
+ return gas_internal_energy_from_entropy(p->rho, entropy);
+}
+
+/**
+ * @brief Returns the pressure of a particle
+ *
+ * @param p The particle of interest
+ * @param dt Time since the last kick
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_pressure(
+ const struct part *restrict p, float dt) {
+
+ const float entropy = p->entropy + p->entropy_dt * dt;
+
+ return gas_pressure_from_entropy(p->rho, entropy);
+}
+
+/**
+ * @brief Returns the entropy of a particle
+ *
+ * @param p The particle of interest
+ * @param dt Time since the last kick
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_entropy(
+ const struct part *restrict p, float dt) {
+
+ return p->entropy + p->entropy_dt * dt;
+}
+
+/**
+ * @brief Returns the sound speed of a particle
+ *
+ * @param p The particle of interest
+ * @param dt Time since the last kick
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_soundspeed(
+ const struct part *restrict p, float dt) {
+
+ return p->force.soundspeed;
+}
+
+/**
+ * @brief Modifies the thermal state of a particle to the imposed internal
+ * energy
+ *
+ * This overrides the current state of the particle but does *not* change its
+ * time-derivatives
+ *
+ * @param p The particle
+ * @param u The new internal energy
+ */
+__attribute__((always_inline)) INLINE static void hydro_set_internal_energy(
+ struct part *restrict p, float u) {
+
+ p->entropy = gas_entropy_from_internal_energy(p->rho, u);
+}
+
+/**
+ * @brief Modifies the thermal state of a particle to the imposed entropy
+ *
+ * This overrides the current state of the particle but does *not* change its
+ * time-derivatives
+ *
+ * @param p The particle
+ * @param S The new entropy
+ */
+__attribute__((always_inline)) INLINE static void hydro_set_entropy(
+ struct part *restrict p, float S) {
+
+ p->entropy = S;
+}
+
+/**
+ * @brief Computes the hydro time-step of a given particle
+ *
+ * @param p Pointer to the particle data
+ * @param xp Pointer to the extended particle data
+ *
+ */
+__attribute__((always_inline)) INLINE static float hydro_compute_timestep(
+ const struct part *restrict p, const struct xpart *restrict xp,
+ const struct hydro_props *restrict hydro_properties) {
+
+ const float CFL_condition = hydro_properties->CFL_condition;
+
+ /* CFL condition */
+ const float dt_cfl =
+ 2.f * kernel_gamma * CFL_condition * p->h / p->force.v_sig;
+
+ return dt_cfl;
+}
+
+/**
+ * @brief Initialises the particles for the first time
+ *
+ * This function is called only once just after the ICs have been
+ * read in to do some conversions.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_first_init_part(
+ struct part *restrict p, struct xpart *restrict xp) {
+
+ p->ti_begin = 0;
+ p->ti_end = 0;
+ xp->v_full[0] = p->v[0];
+ xp->v_full[1] = p->v[1];
+ xp->v_full[2] = p->v[2];
+}
+
+/**
+ * @brief Prepares a particle for the density calculation.
+ *
+ * Zeroes all the relevant arrays in preparation for the sums taking place in
+ * the variaous density tasks
+ *
+ * @param p The particle to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_init_part(
+ struct part *restrict p) {
+ p->density.wcount = 0.f;
+ p->density.wcount_dh = 0.f;
+ p->rho = 0.f;
+ p->rho_dh = 0.f;
+ p->density.div_v = 0.f;
+ p->density.rot_v[0] = 0.f;
+ p->density.rot_v[1] = 0.f;
+ p->density.rot_v[2] = 0.f;
+}
+
+/**
+ * @brief Finishes the density calculation.
+ *
+ * Multiplies the density and number of neighbours by the appropiate constants
+ * and add the self-contribution term.
+ *
+ * @param p The particle to act upon
+ * @param ti_current The current time (on the integer timeline)
+ */
+__attribute__((always_inline)) INLINE static void hydro_end_density(
+ struct part *restrict p, int ti_current) {
+
+ /* Some smoothing length multiples. */
+ const float h = p->h;
+ const float h_inv = 1.0f / h; /* 1/h */
+ const float h_inv_dim = pow_dimension(h_inv); /* 1/h^d */
+ const float h_inv_dim_plus_one = h_inv_dim * h_inv; /* 1/h^(d+1) */
+
+ /* Final operation on the density (add self-contribution). */
+ p->rho += p->mass * kernel_root;
+ p->rho_dh -= hydro_dimension * p->mass * kernel_root;
+ p->density.wcount += kernel_root;
+
+ /* Finish the calculation by inserting the missing h-factors */
+ p->rho *= h_inv_dim;
+ p->rho_dh *= h_inv_dim_plus_one;
+ p->density.wcount *= kernel_norm;
+ p->density.wcount_dh *= h_inv * kernel_gamma * kernel_norm;
+
+ const float irho = 1.f / p->rho;
+
+ /* Compute the derivative term */
+ p->rho_dh = 1.f / (1.f + hydro_dimension_inv * p->h * p->rho_dh * irho);
+
+ /* Finish calculation of the velocity curl components */
+ p->density.rot_v[0] *= h_inv_dim_plus_one * irho;
+ p->density.rot_v[1] *= h_inv_dim_plus_one * irho;
+ p->density.rot_v[2] *= h_inv_dim_plus_one * irho;
+
+ /* Finish calculation of the velocity divergence */
+ p->density.div_v *= h_inv_dim_plus_one * irho;
+}
+
+/**
+ * @brief Prepare a particle for the force calculation.
+ *
+ * Computes viscosity term, conduction term and smoothing length gradient terms.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ * @param ti_current The current time (on the timeline)
+ * @param timeBase The minimal time-step size
+ */
+__attribute__((always_inline)) INLINE static void hydro_prepare_force(
+ struct part *restrict p, struct xpart *restrict xp, int ti_current,
+ double timeBase) {
+
+ const float fac_mu = 1.f; /* Will change with cosmological integration */
+
+ /* Compute the norm of the curl */
+ const float curl_v = sqrtf(p->density.rot_v[0] * p->density.rot_v[0] +
+ p->density.rot_v[1] * p->density.rot_v[1] +
+ p->density.rot_v[2] * p->density.rot_v[2]);
+
+ /* Compute the norm of div v */
+ const float abs_div_v = fabsf(p->density.div_v);
+
+ /* Compute the pressure */
+ const float half_dt = (ti_current - (p->ti_begin + p->ti_end) / 2) * timeBase;
+ const float pressure = hydro_get_pressure(p, half_dt);
+
+ const float irho = 1.f / p->rho;
+
+ /* Divide the pressure by the density and density gradient */
+ const float P_over_rho2 = pressure * irho * irho * p->rho_dh;
+
+ /* Compute the sound speed */
+ const float soundspeed = sqrtf(hydro_gamma * pressure * irho);
+
+ /* Compute the Balsara switch */
+ const float balsara =
+ abs_div_v / (abs_div_v + curl_v + 0.0001f * soundspeed / fac_mu / p->h);
+
+ /* Update variables. */
+ p->force.P_over_rho2 = P_over_rho2;
+ p->force.soundspeed = soundspeed;
+ p->force.balsara = balsara;
+}
+
+/**
+ * @brief Reset acceleration fields of a particle
+ *
+ * Resets all hydro acceleration and time derivative fields in preparation
+ * for the sums taking place in the variaous force tasks
+ *
+ * @param p The particle to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_reset_acceleration(
+ struct part *restrict p) {
+
+ /* Reset the acceleration. */
+ p->a_hydro[0] = 0.0f;
+ p->a_hydro[1] = 0.0f;
+ p->a_hydro[2] = 0.0f;
+
+ /* Reset the time derivatives. */
+ p->entropy_dt = 0.0f;
+ p->force.h_dt = 0.0f;
+
+ /* Reset maximal signal velocity */
+ p->force.v_sig = 0.0f;
+}
+
+/**
+ * @brief Predict additional particle fields forward in time when drifting
+ *
+ * @param p The particle
+ * @param xp The extended data of the particle
+ * @param t0 The time at the start of the drift
+ * @param t1 The time at the end of the drift
+ * @param timeBase The minimal time-step size
+ */
+__attribute__((always_inline)) INLINE static void hydro_predict_extra(
+ struct part *restrict p, const struct xpart *restrict xp, int t0, int t1,
+ double timeBase) {
+
+ /* Drift the pressure */
+ const float dt_entr = (t1 - (p->ti_begin + p->ti_end) / 2) * timeBase;
+ const float pressure = hydro_get_pressure(p, dt_entr);
+
+ const float irho = 1.f / p->rho;
+
+ /* Divide the pressure by the density and density gradient */
+ const float P_over_rho2 = pressure * irho * irho * p->rho_dh;
+
+ /* Compute the new sound speed */
+ const float soundspeed = sqrtf(hydro_gamma * pressure * irho);
+
+ /* Update variables */
+ p->force.P_over_rho2 = P_over_rho2;
+ p->force.soundspeed = soundspeed;
+}
+
+/**
+ * @brief Finishes the force calculation.
+ *
+ * Multiplies the forces and accelerationsby the appropiate constants
+ *
+ * @param p The particle to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_end_force(
+ struct part *restrict p) {
+
+ p->force.h_dt *= p->h * hydro_dimension_inv;
+
+ p->entropy_dt *=
+ 0.5f * hydro_gamma_minus_one * pow_minus_gamma_minus_one(p->rho);
+}
+
+/**
+ * @brief Kick the additional variables
+ *
+ * @param p The particle to act upon
+ * @param xp The particle extended data to act upon
+ * @param dt The time-step for this kick
+ * @param half_dt The half time-step for this kick
+ */
+__attribute__((always_inline)) INLINE static void hydro_kick_extra(
+ struct part *restrict p, struct xpart *restrict xp, float dt,
+ float half_dt) {
+
+ /* Do not decrease the entropy (temperature) by more than a factor of 2*/
+ const float entropy_change = p->entropy_dt * dt;
+ if (entropy_change > -0.5f * p->entropy)
+ p->entropy += entropy_change;
+ else
+ p->entropy *= 0.5f;
+
+ /* Do not 'overcool' when timestep increases */
+ if (p->entropy + p->entropy_dt * half_dt < 0.5f * p->entropy)
+ p->entropy_dt = -0.5f * p->entropy / half_dt;
+}
+
+/**
+ * @brief Converts hydro quantity of a particle at the start of a run
+ *
+ * Requires the density to be known
+ *
+ * @param p The particle to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_convert_quantities(
+ struct part *restrict p) {
+
+ /* We read u in the entropy field. We now get S from u */
+ p->entropy = gas_entropy_from_internal_energy(p->rho, p->entropy);
+}
+
+#endif /* SWIFT_PRESSURE_ENTROPY_HYDRO_H */
diff --git a/src/hydro/PressureEntropy/hydro_debug.h b/src/hydro/PressureEntropy/hydro_debug.h
new file mode 100644
index 0000000000000000000000000000000000000000..f98f9c3ffce1ec0212efdf6663f95d45dd4793f3
--- /dev/null
+++ b/src/hydro/PressureEntropy/hydro_debug.h
@@ -0,0 +1,50 @@
+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Coypright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_PRESSURE_ENTROPY_HYDRO_DEBUG_H
+#define SWIFT_PRESSURE_ENTROPY_HYDRO_DEBUG_H
+
+/**
+ * @file PressureEntropy/hydro_debug.h
+ * @brief Pressure-Entropy implementation of SPH (Particle definition)
+ *
+ * The thermal variable is the entropy (S) and the entropy is smoothed over
+ * contact discontinuities to prevent spurious surface tension.
+ *
+ * Follows Hopkins, P., MNRAS, 2013, Volume 428, Issue 4, pp. 2840-2856
+ */
+
+__attribute__((always_inline)) INLINE static void hydro_debug_particle(
+ const struct part* p, const struct xpart* xp) {
+ printf(
+ "x=[%.3e,%.3e,%.3e], "
+ "v=[%.3e,%.3e,%.3e],v_full=[%.3e,%.3e,%.3e] \n a=[%.3e,%.3e,%.3e],\n "
+ "h=%.3e, wcount=%.3f, wcount_dh=%.3e, m=%.3e, dh_drho=%.3e, rho=%.3e, "
+ "P=%.3e, P_over_rho2=%.3e, S=%.3e, dS/dt=%.3e, c=%.3e\n"
+ "divV=%.3e, rotV=[%.3e,%.3e,%.3e], balsara=%.3e \n "
+ "v_sig=%e dh/dt=%.3e t_begin=%d, t_end=%d\n",
+ p->x[0], p->x[1], p->x[2], p->v[0], p->v[1], p->v[2], xp->v_full[0],
+ xp->v_full[1], xp->v_full[2], p->a_hydro[0], p->a_hydro[1], p->a_hydro[2],
+ p->h, p->density.wcount, p->density.wcount_dh, p->mass, p->rho_dh, p->rho,
+ hydro_get_pressure(p, 0.), p->force.P_over_rho2, p->entropy,
+ p->entropy_dt, p->force.soundspeed, p->density.div_v, p->density.rot_v[0],
+ p->density.rot_v[1], p->density.rot_v[2], p->force.balsara,
+ p->force.v_sig, p->force.h_dt, p->ti_begin, p->ti_end);
+}
+
+#endif /* SWIFT_PRESSURE_ENTROPY_HYDRO_DEBUG_H */
diff --git a/src/hydro/PressureEntropy/hydro_iact.h b/src/hydro/PressureEntropy/hydro_iact.h
new file mode 100644
index 0000000000000000000000000000000000000000..30efe42cfc445169f3954325bae30b7fed2faed2
--- /dev/null
+++ b/src/hydro/PressureEntropy/hydro_iact.h
@@ -0,0 +1,367 @@
+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_PRESSURE_ENTROPY_HYDRO_IACT_H
+#define SWIFT_PRESSURE_ENTROPY_HYDRO_IACT_H
+
+/**
+ * @file PressureEntropy/hydro_part.h
+ * @brief Pressure-Entropy implementation of SPH (Particle definition)
+ *
+ * The thermal variable is the entropy (S) and the entropy is smoothed over
+ * contact discontinuities to prevent spurious surface tension.
+ *
+ * Follows Hopkins, P., MNRAS, 2013, Volume 428, Issue 4, pp. 2840-2856
+ */
+
+/**
+ * @brief Density loop
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_density(
+ float r2, float *dx, float hi, float hj, struct part *pi, struct part *pj) {
+
+ float wi, wi_dx;
+ float wj, wj_dx;
+ float dv[3], curlvr[3];
+
+ /* Get the masses. */
+ const float mi = pi->mass;
+ const float mj = pj->mass;
+
+ /* Get r and r inverse. */
+ const float r = sqrtf(r2);
+ const float r_inv = 1.0f / r;
+
+ /* Compute the kernel function for pi */
+ const float hi_inv = 1.f / hi;
+ const float ui = r * hi_inv;
+ kernel_deval(ui, &wi, &wi_dx);
+
+ /* Compute contribution to the density */
+ pi->rho += mj * wi;
+ pi->rho_dh -= mj * (hydro_dimension * wi + ui * wi_dx);
+
+ /* Compute contribution to the number of neighbours */
+ pi->density.wcount += wi;
+ pi->density.wcount_dh -= ui * wi_dx;
+
+ /* Compute the kernel function for pj */
+ const float hj_inv = 1.f / hj;
+ const float uj = r * hj_inv;
+ kernel_deval(uj, &wj, &wj_dx);
+
+ /* Compute contribution to the density */
+ pj->rho += mi * wj;
+ pj->rho_dh -= mi * (hydro_dimension * wj + uj * wj_dx);
+
+ /* Compute contribution to the number of neighbours */
+ pj->density.wcount += wj;
+ pj->density.wcount_dh -= uj * wj_dx;
+
+ const float faci = mj * wi_dx * r_inv;
+ const float facj = mi * wj_dx * r_inv;
+
+ /* Compute dv dot r */
+ dv[0] = pi->v[0] - pj->v[0];
+ dv[1] = pi->v[1] - pj->v[1];
+ dv[2] = pi->v[2] - pj->v[2];
+ const float dvdr = dv[0] * dx[0] + dv[1] * dx[1] + dv[2] * dx[2];
+
+ pi->density.div_v -= faci * dvdr;
+ pj->density.div_v -= facj * dvdr;
+
+ /* Compute dv cross r */
+ curlvr[0] = dv[1] * dx[2] - dv[2] * dx[1];
+ curlvr[1] = dv[2] * dx[0] - dv[0] * dx[2];
+ curlvr[2] = dv[0] * dx[1] - dv[1] * dx[0];
+
+ pi->density.rot_v[0] += faci * curlvr[0];
+ pi->density.rot_v[1] += faci * curlvr[1];
+ pi->density.rot_v[2] += faci * curlvr[2];
+
+ pj->density.rot_v[0] += facj * curlvr[0];
+ pj->density.rot_v[1] += facj * curlvr[1];
+ pj->density.rot_v[2] += facj * curlvr[2];
+}
+
+/**
+ * @brief Density loop (Vectorized version)
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_vec_density(
+ float *R2, float *Dx, float *Hi, float *Hj, struct part **pi,
+ struct part **pj) {
+
+ error("Vectorized version of Pressure-Entropy SPH routine not existatn yet.");
+}
+
+/**
+ * @brief Density loop (non-symmetric version)
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_nonsym_density(
+ float r2, float *dx, float hi, float hj, struct part *pi, struct part *pj) {
+
+ float wi, wi_dx;
+ float dv[3], curlvr[3];
+
+ /* Get the masses. */
+ const float mj = pj->mass;
+
+ /* Get r and r inverse. */
+ const float r = sqrtf(r2);
+ const float ri = 1.0f / r;
+
+ /* Compute the kernel function */
+ const float h_inv = 1.0f / hi;
+ const float u = r * h_inv;
+ kernel_deval(u, &wi, &wi_dx);
+
+ /* Compute contribution to the density */
+ pi->rho += mj * wi;
+ pi->rho_dh -= mj * (hydro_dimension * wi + u * wi_dx);
+
+ /* Compute contribution to the number of neighbours */
+ pi->density.wcount += wi;
+ pi->density.wcount_dh -= u * wi_dx;
+
+ const float fac = mj * wi_dx * ri;
+
+ /* Compute dv dot r */
+ dv[0] = pi->v[0] - pj->v[0];
+ dv[1] = pi->v[1] - pj->v[1];
+ dv[2] = pi->v[2] - pj->v[2];
+ const float dvdr = dv[0] * dx[0] + dv[1] * dx[1] + dv[2] * dx[2];
+ pi->density.div_v -= fac * dvdr;
+
+ /* Compute dv cross r */
+ curlvr[0] = dv[1] * dx[2] - dv[2] * dx[1];
+ curlvr[1] = dv[2] * dx[0] - dv[0] * dx[2];
+ curlvr[2] = dv[0] * dx[1] - dv[1] * dx[0];
+
+ pi->density.rot_v[0] += fac * curlvr[0];
+ pi->density.rot_v[1] += fac * curlvr[1];
+ pi->density.rot_v[2] += fac * curlvr[2];
+}
+
+/**
+ * @brief Density loop (non-symmetric vectorized version)
+ */
+__attribute__((always_inline)) INLINE static void
+runner_iact_nonsym_vec_density(float *R2, float *Dx, float *Hi, float *Hj,
+ struct part **pi, struct part **pj) {
+
+ error("Vectorized version of Pressure-Entropy SPH routine not existatn yet.");
+}
+
+/**
+ * @brief Force loop
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_force(
+ float r2, float *dx, float hi, float hj, struct part *pi, struct part *pj) {
+
+ float wi, wj, wi_dx, wj_dx;
+
+ const float fac_mu = 1.f; /* Will change with cosmological integration */
+
+ const float r = sqrtf(r2);
+ const float r_inv = 1.0f / r;
+
+ /* Get some values in local variables. */
+ const float mi = pi->mass;
+ const float mj = pj->mass;
+ const float rhoi = pi->rho;
+ const float rhoj = pj->rho;
+
+ /* Get the kernel for hi. */
+ const float hi_inv = 1.0f / hi;
+ const float hid_inv = pow_dimension_plus_one(hi_inv); /* 1/h^(d+1) */
+ const float ui = r * hi_inv;
+ kernel_deval(ui, &wi, &wi_dx);
+ const float wi_dr = hid_inv * wi_dx;
+
+ /* Get the kernel for hj. */
+ const float hj_inv = 1.0f / hj;
+ const float hjd_inv = pow_dimension_plus_one(hj_inv); /* 1/h^(d+1) */
+ const float xj = r * hj_inv;
+ kernel_deval(xj, &wj, &wj_dx);
+ const float wj_dr = hjd_inv * wj_dx;
+
+ /* Compute gradient terms */
+ const float P_over_rho2_i = pi->force.P_over_rho2;
+ const float P_over_rho2_j = pj->force.P_over_rho2;
+
+ /* Compute sound speeds */
+ const float ci = pi->force.soundspeed;
+ const float cj = pj->force.soundspeed;
+
+ /* Compute dv dot r. */
+ const float dvdr = (pi->v[0] - pj->v[0]) * dx[0] +
+ (pi->v[1] - pj->v[1]) * dx[1] +
+ (pi->v[2] - pj->v[2]) * dx[2];
+
+ /* Balsara term */
+ const float balsara_i = pi->force.balsara;
+ const float balsara_j = pj->force.balsara;
+
+ /* Are the particles moving towards each others ? */
+ const float omega_ij = fminf(dvdr, 0.f);
+ const float mu_ij = fac_mu * r_inv * omega_ij; /* This is 0 or negative */
+
+ /* Signal velocity */
+ const float v_sig = ci + cj - 3.f * mu_ij;
+
+ /* Now construct the full viscosity term */
+ const float rho_ij = 0.5f * (rhoi + rhoj);
+ const float visc = -0.25f * const_viscosity_alpha * v_sig * mu_ij *
+ (balsara_i + balsara_j) / rho_ij;
+
+ /* Now, convolve with the kernel */
+ const float visc_term = 0.5f * visc * (wi_dr + wj_dr) * r_inv;
+ const float sph_term =
+ (P_over_rho2_i * wi_dr + P_over_rho2_j * wj_dr) * r_inv;
+
+ /* Eventually got the acceleration */
+ const float acc = visc_term + sph_term;
+
+ /* Use the force Luke ! */
+ pi->a_hydro[0] -= mj * acc * dx[0];
+ pi->a_hydro[1] -= mj * acc * dx[1];
+ pi->a_hydro[2] -= mj * acc * dx[2];
+
+ pj->a_hydro[0] += mi * acc * dx[0];
+ pj->a_hydro[1] += mi * acc * dx[1];
+ pj->a_hydro[2] += mi * acc * dx[2];
+
+ /* Get the time derivative for h. */
+ pi->force.h_dt -= mj * dvdr * r_inv / rhoj * wi_dr;
+ pj->force.h_dt -= mi * dvdr * r_inv / rhoi * wj_dr;
+
+ /* Update the signal velocity. */
+ pi->force.v_sig = fmaxf(pi->force.v_sig, v_sig);
+ pj->force.v_sig = fmaxf(pj->force.v_sig, v_sig);
+
+ /* Change in entropy */
+ pi->entropy_dt += mj * visc_term * dvdr;
+ pj->entropy_dt += mi * visc_term * dvdr;
+}
+
+/**
+ * @brief Force loop (Vectorized version)
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_vec_force(
+ float *R2, float *Dx, float *Hi, float *Hj, struct part **pi,
+ struct part **pj) {
+
+ error("Vectorized version of Pressure-Entropy SPH routine not existatn yet.");
+}
+
+/**
+ * @brief Force loop (non-symmetric version)
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_nonsym_force(
+ float r2, float *dx, float hi, float hj, struct part *pi, struct part *pj) {
+
+ float wi, wj, wi_dx, wj_dx;
+
+ const float fac_mu = 1.f; /* Will change with cosmological integration */
+
+ const float r = sqrtf(r2);
+ const float r_inv = 1.0f / r;
+
+ /* Get some values in local variables. */
+ // const float mi = pi->mass;
+ const float mj = pj->mass;
+ const float rhoi = pi->rho;
+ const float rhoj = pj->rho;
+
+ /* Get the kernel for hi. */
+ const float hi_inv = 1.0f / hi;
+ const float hid_inv = pow_dimension_plus_one(hi_inv); /* 1/h^(d+1) */
+ const float ui = r * hi_inv;
+ kernel_deval(ui, &wi, &wi_dx);
+ const float wi_dr = hid_inv * wi_dx;
+
+ /* Get the kernel for hj. */
+ const float hj_inv = 1.0f / hj;
+ const float hjd_inv = pow_dimension_plus_one(hj_inv); /* 1/h^(d+1) */
+ const float xj = r * hj_inv;
+ kernel_deval(xj, &wj, &wj_dx);
+ const float wj_dr = hjd_inv * wj_dx;
+
+ /* Compute gradient terms */
+ const float P_over_rho2_i = pi->force.P_over_rho2;
+ const float P_over_rho2_j = pj->force.P_over_rho2;
+
+ /* Compute sound speeds */
+ const float ci = pi->force.soundspeed;
+ const float cj = pj->force.soundspeed;
+
+ /* Compute dv dot r. */
+ const float dvdr = (pi->v[0] - pj->v[0]) * dx[0] +
+ (pi->v[1] - pj->v[1]) * dx[1] +
+ (pi->v[2] - pj->v[2]) * dx[2];
+
+ /* Balsara term */
+ const float balsara_i = pi->force.balsara;
+ const float balsara_j = pj->force.balsara;
+
+ /* Are the particles moving towards each others ? */
+ const float omega_ij = fminf(dvdr, 0.f);
+ const float mu_ij = fac_mu * r_inv * omega_ij; /* This is 0 or negative */
+
+ /* Signal velocity */
+ const float v_sig = ci + cj - 3.f * mu_ij;
+
+ /* Now construct the full viscosity term */
+ const float rho_ij = 0.5f * (rhoi + rhoj);
+ const float visc = -0.25f * const_viscosity_alpha * v_sig * mu_ij *
+ (balsara_i + balsara_j) / rho_ij;
+
+ /* Now, convolve with the kernel */
+ const float visc_term = 0.5f * visc * (wi_dr + wj_dr) * r_inv;
+ const float sph_term =
+ (P_over_rho2_i * wi_dr + P_over_rho2_j * wj_dr) * r_inv;
+
+ /* Eventually got the acceleration */
+ const float acc = visc_term + sph_term;
+
+ /* Use the force Luke ! */
+ pi->a_hydro[0] -= mj * acc * dx[0];
+ pi->a_hydro[1] -= mj * acc * dx[1];
+ pi->a_hydro[2] -= mj * acc * dx[2];
+
+ /* Get the time derivative for h. */
+ pi->force.h_dt -= mj * dvdr * r_inv / rhoj * wi_dr;
+
+ /* Update the signal velocity. */
+ pi->force.v_sig = fmaxf(pi->force.v_sig, v_sig);
+
+ /* Change in entropy */
+ pi->entropy_dt += mj * visc_term * dvdr;
+}
+
+/**
+ * @brief Force loop (Vectorized non-symmetric version)
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_nonsym_vec_force(
+ float *R2, float *Dx, float *Hi, float *Hj, struct part **pi,
+ struct part **pj) {
+
+ error("Vectorized version of Pressure-Entropy SPH routine not existatn yet.");
+}
+
+#endif /* SWIFT_PRESSURE_ENTROPY_HYDRO_IACT_H */
diff --git a/src/hydro/PressureEntropy/hydro_io.h b/src/hydro/PressureEntropy/hydro_io.h
new file mode 100644
index 0000000000000000000000000000000000000000..b4e2c3fc3fcc6d14b5be1fc7a6635ced5d9e24ab
--- /dev/null
+++ b/src/hydro/PressureEntropy/hydro_io.h
@@ -0,0 +1,138 @@
+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Coypright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_PRESSURE_ENTROPY_HYDRO_IO_H
+#define SWIFT_PRESSURE_ENTROPY_HYDRO_IO_H
+
+/**
+ * @file PressureEntropy/hydro_io.h
+ * @brief Pressure-Entropy implementation of SPH (Particle definition)
+ *
+ * The thermal variable is the entropy (S) and the entropy is smoothed over
+ * contact discontinuities to prevent spurious surface tension.
+ *
+ * Follows Hopkins, P., MNRAS, 2013, Volume 428, Issue 4, pp. 2840-2856
+ */
+
+#include "adiabatic_index.h"
+#include "hydro.h"
+#include "io_properties.h"
+#include "kernel_hydro.h"
+
+/**
+ * @brief Specifies which particle fields to read from a dataset
+ *
+ * @param parts The particle array.
+ * @param list The list of i/o properties to read.
+ * @param num_fields The number of i/o fields to read.
+ */
+void hydro_read_particles(struct part* parts, struct io_props* list,
+ int* num_fields) {
+
+ *num_fields = 8;
+
+ /* List what we want to read */
+ list[0] = io_make_input_field("Coordinates", DOUBLE, 3, COMPULSORY,
+ UNIT_CONV_LENGTH, parts, x);
+ list[1] = io_make_input_field("Velocities", FLOAT, 3, COMPULSORY,
+ UNIT_CONV_SPEED, parts, v);
+ list[2] = io_make_input_field("Masses", FLOAT, 1, COMPULSORY, UNIT_CONV_MASS,
+ parts, mass);
+ list[3] = io_make_input_field("SmoothingLength", FLOAT, 1, COMPULSORY,
+ UNIT_CONV_LENGTH, parts, h);
+ list[4] = io_make_input_field("InternalEnergy", FLOAT, 1, COMPULSORY,
+ UNIT_CONV_ENERGY_PER_UNIT_MASS, parts, entropy);
+ list[5] = io_make_input_field("ParticleIDs", ULONGLONG, 1, COMPULSORY,
+ UNIT_CONV_NO_UNITS, parts, id);
+ list[6] = io_make_input_field("Accelerations", FLOAT, 3, OPTIONAL,
+ UNIT_CONV_ACCELERATION, parts, a_hydro);
+ list[7] = io_make_input_field("Density", FLOAT, 1, OPTIONAL,
+ UNIT_CONV_DENSITY, parts, rho);
+}
+
+float convert_u(struct engine* e, struct part* p) {
+
+ return hydro_get_internal_energy(p, 0);
+}
+
+float convert_P(struct engine* e, struct part* p) {
+
+ return hydro_get_pressure(p, 0);
+}
+
+/**
+ * @brief Specifies which particle fields to write to a dataset
+ *
+ * @param parts The particle array.
+ * @param list The list of i/o properties to write.
+ * @param num_fields The number of i/o fields to write.
+ */
+void hydro_write_particles(struct part* parts, struct io_props* list,
+ int* num_fields) {
+
+ *num_fields = 10;
+
+ /* List what we want to write */
+ list[0] = io_make_output_field("Coordinates", DOUBLE, 3, UNIT_CONV_LENGTH,
+ parts, x);
+ list[1] =
+ io_make_output_field("Velocities", FLOAT, 3, UNIT_CONV_SPEED, parts, v);
+ list[2] =
+ io_make_output_field("Masses", FLOAT, 1, UNIT_CONV_MASS, parts, mass);
+ list[3] = io_make_output_field("SmoothingLength", FLOAT, 1, UNIT_CONV_LENGTH,
+ parts, h);
+ list[4] = io_make_output_field_convert_part("InternalEnergy", FLOAT, 1,
+ UNIT_CONV_ENERGY_PER_UNIT_MASS,
+ parts, entropy, convert_u);
+ list[5] = io_make_output_field("ParticleIDs", ULONGLONG, 1,
+ UNIT_CONV_NO_UNITS, parts, id);
+ list[6] = io_make_output_field("Acceleration", FLOAT, 3,
+ UNIT_CONV_ACCELERATION, parts, a_hydro);
+ list[7] =
+ io_make_output_field("Density", FLOAT, 1, UNIT_CONV_DENSITY, parts, rho);
+ list[8] = io_make_output_field("Entropy", FLOAT, 1,
+ UNIT_CONV_ENTROPY_PER_UNIT_MASS, parts, rho);
+ list[9] = io_make_output_field_convert_part(
+ "Pressure", FLOAT, 1, UNIT_CONV_PRESSURE, parts, rho, convert_P);
+}
+
+/**
+ * @brief Writes the current model of SPH to the file
+ * @param h_grpsph The HDF5 group in which to write
+ */
+void writeSPHflavour(hid_t h_grpsph) {
+
+ /* Viscosity and thermal conduction */
+ /* Nothing in this minimal model... */
+ writeAttribute_s(h_grpsph, "Thermal Conductivity Model", "No treatment");
+ writeAttribute_s(h_grpsph, "Viscosity Model",
+ "Minimal treatment as in Monaghan (1992)");
+
+ /* Time integration properties */
+ writeAttribute_f(h_grpsph, "Maximal Delta u change over dt",
+ const_max_u_change);
+}
+
+/**
+ * @brief Are we writing entropy in the internal energy field ?
+ *
+ * @return 1 if entropy is in 'internal energy', 0 otherwise.
+ */
+int writeEntropyFlag() { return 0; }
+
+#endif /* SWIFT_PRESSURE_ENTROPY_HYDRO_IO_H */
diff --git a/src/hydro/PressureEntropy/hydro_part.h b/src/hydro/PressureEntropy/hydro_part.h
new file mode 100644
index 0000000000000000000000000000000000000000..0591c1829bfebe703a559d308dbed5574e705d2e
--- /dev/null
+++ b/src/hydro/PressureEntropy/hydro_part.h
@@ -0,0 +1,125 @@
+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_PRESSURE_ENTROPY_HYDRO_PART_H
+#define SWIFT_PRESSURE_ENTROPY_HYDRO_PART_H
+
+/**
+ * @file PressureEntropy/hydro_part.h
+ * @brief Pressure-Entropy implementation of SPH (Particle definition)
+ *
+ * The thermal variable is the entropy (S) and the entropy is smoothed over
+ * contact discontinuities to prevent spurious surface tension.
+ *
+ * Follows Hopkins, P., MNRAS, 2013, Volume 428, Issue 4, pp. 2840-2856
+ */
+
+/* Extra particle data not needed during the SPH loops over neighbours. */
+struct xpart {
+
+ /* Offset between current position and position at last tree rebuild. */
+ float x_diff[3];
+
+ /* Velocity at the last full step. */
+ float v_full[3];
+
+} __attribute__((aligned(xpart_align)));
+
+/* Data of a single particle. */
+struct part {
+
+ /* Particle position. */
+ double x[3];
+
+ /* Particle predicted velocity. */
+ float v[3];
+
+ /* Particle acceleration. */
+ float a_hydro[3];
+
+ /* Particle cutoff radius. */
+ float h;
+
+ /* Particle mass. */
+ float mass;
+
+ /* Particle time of beginning of time-step. */
+ int ti_begin;
+
+ /* Particle time of end of time-step. */
+ int ti_end;
+
+ /* Particle density. */
+ float rho;
+
+ /* Particle entropy. */
+ float entropy;
+
+ /* Entropy time derivative */
+ float entropy_dt;
+
+ /* Derivative of the density with respect to smoothing length. */
+ float rho_dh;
+
+ union {
+
+ struct {
+
+ /* Number of neighbours. */
+ float wcount;
+
+ /* Number of neighbours spatial derivative. */
+ float wcount_dh;
+
+ /* Particle velocity curl. */
+ float rot_v[3];
+
+ /* Particle velocity divergence. */
+ float div_v;
+
+ } density;
+
+ struct {
+
+ /* Balsara switch */
+ float balsara;
+
+ /* Pressure over density squared (including drho/dh term) */
+ float P_over_rho2;
+
+ /* Particle sound speed. */
+ float soundspeed;
+
+ /* Signal velocity. */
+ float v_sig;
+
+ /* Time derivative of the smoothing length */
+ float h_dt;
+
+ } force;
+ };
+
+ /* Particle ID. */
+ long long id;
+
+ /* Pointer to corresponding gravity part. */
+ struct gpart* gpart;
+
+} __attribute__((aligned(part_align)));
+
+#endif /* SWIFT_PRESSURE_ENTROPY_HYDRO_PART_H */
diff --git a/src/hydro_io.h b/src/hydro_io.h
index 30d663f647c9b763e9b19177e9ba8ef374855768..5ca09635e20cbd20e677ae5d7a390bfb38792af4 100644
--- a/src/hydro_io.h
+++ b/src/hydro_io.h
@@ -26,6 +26,8 @@
#include "./hydro/Minimal/hydro_io.h"
#elif defined(GADGET2_SPH)
#include "./hydro/Gadget2/hydro_io.h"
+#elif defined(HOPKINS_PE_SPH)
+#include "./hydro/PressureEntropy/hydro_io.h"
#elif defined(DEFAULT_SPH)
#include "./hydro/Default/hydro_io.h"
#else
diff --git a/src/part.h b/src/part.h
index efca7b6b5bef49f20df1e2c45b30f65ecbbf4960..188330ebae83299ceb69f2ea755b0289304a248f 100644
--- a/src/part.h
+++ b/src/part.h
@@ -43,6 +43,8 @@
#include "./hydro/Minimal/hydro_part.h"
#elif defined(GADGET2_SPH)
#include "./hydro/Gadget2/hydro_part.h"
+#elif defined(HOPKINS_PE_SPH)
+#include "./hydro/PressureEntropy/hydro_part.h"
#elif defined(DEFAULT_SPH)
#include "./hydro/Default/hydro_part.h"
#else