diff --git a/examples/main.c b/examples/main.c
index c88f92a07a747c327692b5e0fbbc7dc07b93ac0c..2b33a12eb8737b3dfb7ef35c0cdc10df5ab5ac82 100644
--- a/examples/main.c
+++ b/examples/main.c
@@ -1,3 +1,4 @@
+
 /*******************************************************************************
  * This file is part of SWIFT.
  * Copyright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk),
@@ -79,7 +80,9 @@ int main(int argc, char *argv[]) {
   int nr_nodes = 1, myrank = 0;
   FILE *file_thread;
   int with_outputs = 1;
-  int verbose = 0, talking;
+  int with_gravity = 0;
+  int engine_policies = 0;
+  int verbose = 0, talking = 0;
   unsigned long long cpufreq = 0;
 
 #ifdef WITH_MPI
@@ -97,8 +100,11 @@ int main(int argc, char *argv[]) {
 #endif
 #endif
 
-/* Choke on FP-exceptions. */
-// feenableexcept( FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW );
+  /* Choke on FP-exceptions. */
+  // feenableexcept( FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW );
+
+  /* Initialize CPU frequency, this also starts time. */
+  clocks_set_cpufreq(cpufreq);
 
 #ifdef WITH_MPI
   /* Start by initializing MPI. */
@@ -128,9 +134,6 @@ int main(int argc, char *argv[]) {
          &initial_partition.grid[1], &initial_partition.grid[0]);
 #endif
 
-  /* Initialize CPU frequency, this also starts time. */
-  clocks_set_cpufreq(cpufreq);
-
   /* Greeting message */
   if (myrank == 0) greetings();
 
@@ -156,7 +159,7 @@ int main(int argc, char *argv[]) {
   bzero(&s, sizeof(struct space));
 
   /* Parse the options */
-  while ((c = getopt(argc, argv, "a:c:d:e:f:h:m:oP:q:R:s:t:v:w:y:z:")) != -1)
+  while ((c = getopt(argc, argv, "a:c:d:e:f:gh:m:oP:q:R:s:t:v:w:y:z:")) != -1)
     switch (c) {
       case 'a':
         if (sscanf(optarg, "%lf", &scaling) != 1)
@@ -185,6 +188,9 @@ int main(int argc, char *argv[]) {
       case 'f':
         if (!strcpy(ICfileName, optarg)) error("Error parsing IC file name.");
         break;
+      case 'g':
+        with_gravity = 1;
+        break;
       case 'h':
         if (sscanf(optarg, "%llu", &cpufreq) != 1)
           error("Error parsing CPU frequency.");
@@ -359,8 +365,8 @@ int main(int argc, char *argv[]) {
   read_ic_parallel(ICfileName, dim, &parts, &Ngas, &periodic, myrank, nr_nodes,
                    MPI_COMM_WORLD, MPI_INFO_NULL);
 #else
-  read_ic_serial(ICfileName, dim, &parts, &Ngas, &periodic, myrank, nr_nodes,
-                 MPI_COMM_WORLD, MPI_INFO_NULL);
+  read_ic_serial(ICfileName, dim, &parts, &gparts, &Ngas, &Ngpart, &periodic,
+                 myrank, nr_nodes, MPI_COMM_WORLD, MPI_INFO_NULL);
 #endif
 #else
   read_ic_single(ICfileName, dim, &parts, &gparts, &Ngas, &Ngpart, &periodic);
@@ -376,6 +382,7 @@ int main(int argc, char *argv[]) {
 #if defined(WITH_MPI)
   long long N_long[2] = {Ngas, Ngpart};
   MPI_Reduce(&N_long, &N_total, 2, MPI_LONG_LONG, MPI_SUM, 0, MPI_COMM_WORLD);
+  N_total[1] -= N_total[0];
   if (myrank == 0)
     message("Read %lld gas particles and %lld DM particles from the ICs",
             N_total[0], N_total[1]);
@@ -383,9 +390,34 @@ int main(int argc, char *argv[]) {
   N_total[0] = Ngas;
   N_total[1] = Ngpart - Ngas;
   message("Read %lld gas particles and %lld DM particles from the ICs",
-	  N_total[0], N_total[1]);
+          N_total[0], N_total[1]);
 #endif
 
+  /* MATTHIEU: Temporary fix to preserve master */
+  if (!with_gravity) {
+    free(gparts);
+    for(size_t k = 0; k < Ngas; ++k)
+      parts[k].gpart = NULL;
+    Ngpart = 0;
+#if defined(WITH_MPI)
+    N_long[0] = Ngas;
+    N_long[1] = Ngpart;
+    MPI_Reduce(&N_long, &N_total, 2, MPI_LONG_LONG, MPI_SUM, 0, MPI_COMM_WORLD);
+    if (myrank == 0)
+      message(
+          "AFTER FIX: Read %lld gas particles and %lld DM particles from the "
+          "ICs",
+          N_total[0], N_total[1]);
+#else
+    N_total[0] = Ngas;
+    N_total[1] = Ngpart;
+    message(
+        "AFTER FIX: Read %lld gas particles and %lld DM particles from the ICs",
+        N_total[0], N_total[1]);
+#endif
+  }
+  /* MATTHIEU: End temporary fix */
+
   /* Apply h scaling */
   if (scaling != 1.0)
     for (size_t k = 0; k < Ngas; k++) parts[k].h *= scaling;
@@ -448,12 +480,15 @@ int main(int argc, char *argv[]) {
     message("nr of cells at depth %i is %i.", data[0], data[1]);
   }
 
+  /* Construct the engine policy */
+  engine_policies = ENGINE_POLICY | engine_policy_steal | engine_policy_hydro;
+  if (with_gravity) engine_policies |= engine_policy_external_gravity;
+
   /* Initialize the engine with this space. */
   if (myrank == 0) clocks_gettime(&tic);
   if (myrank == 0) message("nr_nodes is %i.", nr_nodes);
   engine_init(&e, &s, dt_max, nr_threads, nr_queues, nr_nodes, myrank,
-              ENGINE_POLICY | engine_policy_steal | engine_policy_hydro, 0,
-              time_end, dt_min, dt_max, talking);
+              engine_policies, 0, time_end, dt_min, dt_max, talking);
   if (myrank == 0 && verbose) {
     clocks_gettime(&toc);
     message("engine_init took %.3f %s.", clocks_diff(&tic, &toc),
diff --git a/src/common_io.c b/src/common_io.c
index f6a4803333581b69671e3adc223b46122ec5364c..4be334ccd602c7b0c450f5dbe4aedd9155b35ff1 100644
--- a/src/common_io.c
+++ b/src/common_io.c
@@ -489,7 +489,8 @@ void prepare_dm_gparts(struct gpart* gparts, size_t Ndm) {
   for (size_t i = 0; i < Ndm; ++i) {
 
     /* 0 or negative ids are not allowed */
-    if (gparts[i].id <= 0) error("0 or negative ID for DM particle");
+    if (gparts[i].id <= 0)
+      error("0 or negative ID for DM particle %zd: ID=%lld", i, gparts[i].id);
 
     gparts[i].id = -gparts[i].id;
   }
diff --git a/src/common_io.h b/src/common_io.h
index 2623a03f9a25ce0e650dde4f698da6eb49177e26..426fa6a01ec87a4413eceaaeb0d0880cbef8a214 100644
--- a/src/common_io.h
+++ b/src/common_io.h
@@ -70,6 +70,9 @@ enum PARTICLE_TYPE {
   NUM_PARTICLE_TYPES
 };
 
+#define FILENAME_BUFFER_SIZE 150
+#define PARTICLE_GROUP_BUFFER_SIZE 20
+
 hid_t hdf5Type(enum DATA_TYPE type);
 size_t sizeOfType(enum DATA_TYPE type);
 
diff --git a/src/engine.c b/src/engine.c
index b7658535335bd02d309c9cf69da61ffcc2f6c160..01476fe7b0c92cee68594f1baaa5d769d23b29b3 100644
--- a/src/engine.c
+++ b/src/engine.c
@@ -743,7 +743,7 @@ int engine_exchange_strays(struct engine *e, int offset, size_t *ind,
  *neighbours
  *
  * Here we construct all the tasks for all possible neighbouring non-empty
- * local cells in the hierarchy. No dependencies are being added thus far. 
+ * local cells in the hierarchy. No dependencies are being added thus far.
  * Additional loop over neighbours can later be added by simply duplicating
  * all the tasks created by this function.
  *
@@ -1974,6 +1974,7 @@ void engine_init(struct engine *e, struct space *s, float dt, int nr_threads,
   e->dt_max = dt_max;
   e->file_stats = NULL;
   e->verbose = verbose;
+  e->count_step = 0;
   e->wallclock_time = 0.f;
   engine_rank = nodeID;
 
diff --git a/src/gravity/Default/gravity_part.h b/src/gravity/Default/gravity_part.h
index 7ce7b81892582f2a90f7dd07f7f244c0d4ed8afb..aaaab2055d8ce05d77a37d3f40e096bd91d87926 100644
--- a/src/gravity/Default/gravity_part.h
+++ b/src/gravity/Default/gravity_part.h
@@ -50,4 +50,4 @@ struct gpart {
     struct part* part;
   };
 
-} __attribute__((aligned(part_align)));
+} __attribute__((aligned(gpart_align)));
diff --git a/src/serial_io.c b/src/serial_io.c
index 8e63db5cfad3a3b50fc7e350bbac6ce09708230a..3fb1a1d1b743e48be4e8e6d81413c28e8034b870 100644
--- a/src/serial_io.c
+++ b/src/serial_io.c
@@ -68,7 +68,8 @@
  */
 void readArrayBackEnd(hid_t grp, char* name, enum DATA_TYPE type, int N,
                       int dim, long long N_total, long long offset,
-                      char* part_c, enum DATA_IMPORTANCE importance) {
+                      char* part_c, size_t partSize,
+                      enum DATA_IMPORTANCE importance) {
   hid_t h_data = 0, h_err = 0, h_type = 0, h_memspace = 0, h_filespace = 0;
   hsize_t shape[2], offsets[2];
   htri_t exist = 0;
@@ -76,7 +77,6 @@ void readArrayBackEnd(hid_t grp, char* name, enum DATA_TYPE type, int N,
   int i = 0, rank = 0;
   const size_t typeSize = sizeOfType(type);
   const size_t copySize = typeSize * dim;
-  const size_t partSize = sizeof(struct part);
   char* temp_c = 0;
 
   /* Check whether the dataspace exists or not */
@@ -270,7 +270,7 @@ void prepareArray(hid_t grp, char* fileName, FILE* xmfFile, char* name,
 void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
                        enum DATA_TYPE type, int N, int dim, long long N_total,
                        int mpi_rank, long long offset, char* part_c,
-                       struct UnitSystem* us,
+                       size_t partSize, struct UnitSystem* us,
                        enum UnitConversionFactor convFactor) {
 
   hid_t h_data = 0, h_err = 0, h_memspace = 0, h_filespace = 0;
@@ -279,7 +279,6 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
   int i = 0, rank = 0;
   const size_t typeSize = sizeOfType(type);
   const size_t copySize = typeSize * dim;
-  const size_t partSize = sizeof(struct part);
   char* temp_c = 0;
 
   /* message("Writing '%s' array...", name); */
@@ -362,7 +361,7 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
 #define readArray(grp, name, type, N, dim, part, N_total, offset, field, \
                   importance)                                            \
   readArrayBackEnd(grp, name, type, N, dim, N_total, offset,             \
-                   (char*)(&(part[0]).field), importance)
+                   (char*)(&(part[0]).field), sizeof(part[0]), importance)
 
 /**
  * @brief A helper macro to call the readArrayBackEnd function more easily.
@@ -385,20 +384,28 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
 #define writeArray(grp, fileName, xmfFile, name, type, N, dim, part, N_total, \
                    mpi_rank, offset, field, us, convFactor)                   \
   writeArrayBackEnd(grp, fileName, xmfFile, name, type, N, dim, N_total,      \
-                    mpi_rank, offset, (char*)(&(part[0]).field), us,          \
-                    convFactor)
+                    mpi_rank, offset, (char*)(&(part[0]).field),              \
+                    sizeof(part[0]), us, convFactor)
 
 /* Import the right hydro definition */
 #include "hydro_io.h"
+/* Import the right gravity definition */
+#include "gravity_io.h"
 
 /**
  * @brief Reads an HDF5 initial condition file (GADGET-3 type)
  *
  * @param fileName The file to read.
  * @param dim (output) The dimension of the volume read from the file.
- * @param parts (output) The array of #part read from the file.
- * @param N (output) The number of particles read from the file.
+ * @param parts (output) The array of #part (gas particles) read from the file.
+ * @param gparts (output) The array of #gpart read from the file.
+ * @param Ngas (output) The number of #part read from the file on that node.
+ * @param Ngparts (output) The number of #gpart read from the file on that node.
  * @param periodic (output) 1 if the volume is periodic, 0 if not.
+ * @param mpi_rank The MPI rank of this node
+ * @param mpi_size The number of MPI ranks
+ * @param comm The MPI communicator
+ * @param info The MPI information object
  *
  * Opens the HDF5 file fileName and reads the particles contained
  * in the parts array. N is the returned number of particles found
@@ -411,17 +418,18 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
  *
  */
 void read_ic_serial(char* fileName, double dim[3], struct part** parts,
-                    size_t* N, int* periodic, int mpi_rank, int mpi_size,
-                    MPI_Comm comm, MPI_Info info) {
+                    struct gpart** gparts, size_t* Ngas, size_t* Ngparts,
+                    int* periodic, int mpi_rank, int mpi_size, MPI_Comm comm,
+                    MPI_Info info) {
   hid_t h_file = 0, h_grp = 0;
-  double boxSize[3] = {0.0, -1.0, -1.0};
   /* GADGET has only cubic boxes (in cosmological mode) */
-  int numParticles[6] = {0};
-  /* GADGET has 6 particle types. We only keep the type 0*/
-  int numParticles_highWord[6] = {0};
-  long long offset = 0;
-  long long N_total = 0;
-  int rank;
+  double boxSize[3] = {0.0, -1.0, -1.0};
+  /* GADGET has 6 particle types. We only keep the type 0 & 1 for now*/
+  int numParticles[NUM_PARTICLE_TYPES] = {0};
+  int numParticles_highWord[NUM_PARTICLE_TYPES] = {0};
+  size_t N[NUM_PARTICLE_TYPES] = {0};
+  long long N_total[NUM_PARTICLE_TYPES] = {0};
+  long long offset[NUM_PARTICLE_TYPES] = {0};
 
   /* First read some information about the content */
   if (mpi_rank == 0) {
@@ -453,8 +461,10 @@ void read_ic_serial(char* fileName, double dim[3], struct part** parts,
     readAttribute(h_grp, "NumPart_Total", UINT, numParticles);
     readAttribute(h_grp, "NumPart_Total_HighWord", UINT, numParticles_highWord);
 
-    N_total = ((long long)numParticles[0]) +
-              ((long long)numParticles_highWord[0] << 32);
+    for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ++ptype)
+      N_total[ptype] = ((long long)numParticles[ptype]) +
+                       ((long long)numParticles_highWord[ptype] << 32);
+
     dim[0] = boxSize[0];
     dim[1] = (boxSize[1] < 0) ? boxSize[0] : boxSize[1];
     dim[2] = (boxSize[2] < 0) ? boxSize[0] : boxSize[2];
@@ -474,22 +484,38 @@ void read_ic_serial(char* fileName, double dim[3], struct part** parts,
 
   /* Now need to broadcast that information to all ranks. */
   MPI_Bcast(periodic, 1, MPI_INT, 0, comm);
-  MPI_Bcast(&N_total, 1, MPI_LONG_LONG, 0, comm);
+  MPI_Bcast(&N_total, NUM_PARTICLE_TYPES, MPI_LONG_LONG, 0, comm);
   MPI_Bcast(dim, 3, MPI_DOUBLE, 0, comm);
 
   /* Divide the particles among the tasks. */
-  offset = mpi_rank * N_total / mpi_size;
-  *N = (mpi_rank + 1) * N_total / mpi_size - offset;
+  for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ++ptype) {
+    offset[ptype] = mpi_rank * N_total[ptype] / mpi_size;
+    N[ptype] = (mpi_rank + 1) * N_total[ptype] / mpi_size - offset[ptype];
+  }
 
-  /* Allocate memory to store particles */
-  if (posix_memalign((void*)parts, part_align, (*N) * sizeof(struct part)) != 0)
+  /* Allocate memory to store SPH particles */
+  *Ngas = N[0];
+  if (posix_memalign((void*)parts, part_align, (*Ngas) * sizeof(struct part)) !=
+      0)
     error("Error while allocating memory for particles");
-  bzero(*parts, *N * sizeof(struct part));
+  bzero(*parts, *Ngas * sizeof(struct part));
+
+  /* Allocate memory to store all particles */
+  const size_t Ndm = N[1];
+  *Ngparts = N[1] + N[0];
+  if (posix_memalign((void*)gparts, gpart_align,
+                     *Ngparts * sizeof(struct gpart)) != 0)
+    error("Error while allocating memory for gravity particles");
+  bzero(*gparts, *Ngparts * sizeof(struct gpart));
+
   /* message("Allocated %8.2f MB for particles.", *N * sizeof(struct part) / */
   /* 	  (1024.*1024.)); */
 
+  /* message("BoxSize = %lf", dim[0]); */
+  /* message("NumPart = [%zd, %zd] Total = %zd", *Ngas, Ndm, *Ngparts); */
+
   /* Now loop over ranks and read the data */
-  for (rank = 0; rank < mpi_size; ++rank) {
+  for (int rank = 0; rank < mpi_size; ++rank) {
 
     /* Is it this rank's turn to read ? */
     if (rank == mpi_rank) {
@@ -498,17 +524,41 @@ void read_ic_serial(char* fileName, double dim[3], struct part** parts,
       if (h_file < 0)
         error("Error while opening file '%s' on rank %d.", fileName, mpi_rank);
 
-      /* Open SPH particles group */
-      /* message("Reading particle arrays..."); */
-      h_grp = H5Gopen(h_file, "/PartType0", H5P_DEFAULT);
-      if (h_grp < 0)
-        error("Error while opening particle group on rank %d.\n", mpi_rank);
+      /* Loop over all particle types */
+      for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ptype++) {
 
-      /* Read particle fields into the particle structure */
-      hydro_read_particles(h_grp, *N, N_total, offset, *parts);
+        /* Don't do anything if no particle of this kind */
+        if (N[ptype] == 0) continue;
 
-      /* Close particle group */
-      H5Gclose(h_grp);
+        /* Open the particle group in the file */
+        char partTypeGroupName[PARTICLE_GROUP_BUFFER_SIZE];
+        snprintf(partTypeGroupName, PARTICLE_GROUP_BUFFER_SIZE, "/PartType%d",
+                 ptype);
+        h_grp = H5Gopen(h_file, partTypeGroupName, H5P_DEFAULT);
+        if (h_grp < 0) {
+          error("Error while opening particle group %s.", partTypeGroupName);
+        }
+
+        /* Read particle fields into the particle structure */
+        switch (ptype) {
+
+          case GAS:
+            hydro_read_particles(h_grp, N[ptype], N_total[ptype], offset[ptype],
+                                 *parts);
+            break;
+
+          case DM:
+            darkmatter_read_particles(h_grp, N[ptype], N_total[ptype],
+                                      offset[ptype], *gparts);
+            break;
+
+          default:
+            error("Particle Type %d not yet supported. Aborting", ptype);
+        }
+
+        /* Close particle group */
+        H5Gclose(h_grp);
+      }
 
       /* Close file */
       H5Fclose(h_file);
@@ -518,6 +568,12 @@ void read_ic_serial(char* fileName, double dim[3], struct part** parts,
     MPI_Barrier(comm);
   }
 
+  /* Prepare the DM particles */
+  prepare_dm_gparts(*gparts, Ndm);
+
+  /* Now duplicate the hydro particle into gparts */
+  duplicate_hydro_gparts(*parts, *gparts, *Ngas, Ndm);
+
   /* message("Done Reading particles..."); */
 }
 
@@ -525,7 +581,11 @@ void read_ic_serial(char* fileName, double dim[3], struct part** parts,
  * @brief Writes an HDF5 output file (GADGET-3 type) with its XMF descriptor
  *
  * @param e The engine containing all the system.
- * @param us The UnitSystem used for the conversion of units in the output
+ * @param us The UnitSystem used for the conversion of units in the output.
+ * @param mpi_rank The MPI rank of this node.
+ * @param mpi_size The number of MPI ranks.
+ * @param comm The MPI communicator.
+ * @param info The MPI information object
  *
  * Creates an HDF5 output file and writes the particles contained
  * in the engine. If such a file already exists, it is erased and replaced
@@ -538,35 +598,40 @@ void read_ic_serial(char* fileName, double dim[3], struct part** parts,
 void write_output_serial(struct engine* e, struct UnitSystem* us, int mpi_rank,
                          int mpi_size, MPI_Comm comm, MPI_Info info) {
   hid_t h_file = 0, h_grp = 0, h_grpsph = 0;
-  int N = e->s->nr_parts;
+  const size_t Ngas = e->s->nr_parts;
+  const size_t Ntot = e->s->nr_gparts;
   int periodic = e->s->periodic;
-  int numParticles[6] = {N, 0};
-  int numParticlesHighWord[6] = {0};
-  unsigned int flagEntropy[6] = {0};
-  long long N_total = 0, offset = 0;
-  double offset_d = 0., N_d = 0., N_total_d = 0.;
   int numFiles = 1;
-  int rank = 0;
   struct part* parts = e->s->parts;
-  FILE* xmfFile = 0;
+  struct gpart* gparts = e->s->gparts;
+  struct gpart* dmparts = NULL;
   static int outputCount = 0;
+  FILE* xmfFile = 0;
 
+  /* Number of particles of each type */
+  //const size_t Ndm = Ntot - Ngas;
+
+  /* MATTHIEU: Temporary fix to preserve master */
+  const size_t Ndm = Ntot > 0 ? Ntot - Ngas: 0;
+  /* MATTHIEU: End temporary fix */
+  
   /* File name */
-  char fileName[200];
-  sprintf(fileName, "output_%03i.hdf5", outputCount);
+  char fileName[FILENAME_BUFFER_SIZE];
+  snprintf(fileName, FILENAME_BUFFER_SIZE, "output_%03i.hdf5", outputCount);
 
   /* Compute offset in the file and total number of particles */
-  /* Done using double to allow for up to 2^50=10^15 particles */
-  N_d = (double)N;
-  MPI_Exscan(&N_d, &offset_d, 1, MPI_DOUBLE, MPI_SUM, comm);
-  N_total_d = offset_d + N_d;
-  MPI_Bcast(&N_total_d, 1, MPI_DOUBLE, mpi_size - 1, comm);
-  if (N_total_d > 1.e15)
-    error(
-        "Error while computing the offset for parallel output: Simulation has "
-        "more than 10^15 particles.\n");
-  N_total = (long long)N_total_d;
-  offset = (long long)offset_d;
+  size_t N[NUM_PARTICLE_TYPES] = {Ngas, Ndm, 0};
+  long long N_total[NUM_PARTICLE_TYPES] = {0};
+  long long offset[NUM_PARTICLE_TYPES] = {0};
+  MPI_Exscan(&N, &offset, NUM_PARTICLE_TYPES, MPI_LONG_LONG, MPI_SUM, comm);
+  for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ++ptype)
+    N_total[ptype] = offset[ptype] + N[ptype];
+
+  /* The last rank now has the correct N_total. Let's broadcast from there */
+  MPI_Bcast(&N_total, 6, MPI_LONG_LONG, mpi_size - 1, comm);
+
+  /* Now everybody konws its offset and the total number of particles of each
+   * type */
 
   /* Do common stuff first */
   if (mpi_rank == 0) {
@@ -578,7 +643,7 @@ void write_output_serial(struct engine* e, struct UnitSystem* us, int mpi_rank,
     xmfFile = prepareXMFfile();
 
     /* Write the part corresponding to this specific output */
-    writeXMFheader(xmfFile, N_total, fileName, e->time);
+    // writeXMFheader(xmfFile, N_total, fileName, e->time);
 
     /* Open file */
     /* message("Opening file '%s'.", fileName); */
@@ -610,15 +675,24 @@ void write_output_serial(struct engine* e, struct UnitSystem* us, int mpi_rank,
     writeAttribute(h_grp, "Time", DOUBLE, &dblTime, 1);
 
     /* GADGET-2 legacy values */
-    numParticles[0] = (unsigned int)N_total;
-    writeAttribute(h_grp, "NumPart_ThisFile", UINT, numParticles, 6);
-    writeAttribute(h_grp, "NumPart_Total", UINT, numParticles, 6);
-    numParticlesHighWord[0] = (unsigned int)(N_total >> 32);
+    /* Number of particles of each type */
+    unsigned int numParticles[NUM_PARTICLE_TYPES] = {0};
+    unsigned int numParticlesHighWord[NUM_PARTICLE_TYPES] = {0};
+    for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ++ptype) {
+      numParticles[ptype] = (unsigned int)N_total[ptype];
+      numParticlesHighWord[ptype] = (unsigned int)(N_total[ptype] >> 32);
+    }
+    writeAttribute(h_grp, "NumPart_ThisFile", LONGLONG, N_total,
+                   NUM_PARTICLE_TYPES);
+    writeAttribute(h_grp, "NumPart_Total", UINT, numParticles,
+                   NUM_PARTICLE_TYPES);
     writeAttribute(h_grp, "NumPart_Total_HighWord", UINT, numParticlesHighWord,
-                   6);
+                   NUM_PARTICLE_TYPES);
     double MassTable[6] = {0., 0., 0., 0., 0., 0.};
-    writeAttribute(h_grp, "MassTable", DOUBLE, MassTable, 6);
-    writeAttribute(h_grp, "Flag_Entropy_ICs", UINT, flagEntropy, 6);
+    writeAttribute(h_grp, "MassTable", DOUBLE, MassTable, NUM_PARTICLE_TYPES);
+    unsigned int flagEntropy[NUM_PARTICLE_TYPES] = {0};
+    writeAttribute(h_grp, "Flag_Entropy_ICs", UINT, flagEntropy,
+                   NUM_PARTICLE_TYPES);
     writeAttribute(h_grp, "NumFilesPerSnapshot", INT, &numFiles, 1);
 
     /* Close header */
@@ -636,21 +710,32 @@ void write_output_serial(struct engine* e, struct UnitSystem* us, int mpi_rank,
     /* Print the system of Units */
     writeUnitSystem(h_file, us);
 
-    /* Create SPH particles group */
-    /* message("Writing particle arrays..."); */
-    h_grp =
-        H5Gcreate(h_file, "/PartType0", H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
-    if (h_grp < 0) error("Error while creating particle group.\n");
+    /* Loop over all particle types */
+    for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ptype++) {
 
-    /* Close particle group */
-    H5Gclose(h_grp);
+      /* Don't do anything if no particle of this kind */
+      if (N_total[ptype] == 0) continue;
+
+      /* Open the particle group in the file */
+      char partTypeGroupName[PARTICLE_GROUP_BUFFER_SIZE];
+      snprintf(partTypeGroupName, PARTICLE_GROUP_BUFFER_SIZE, "/PartType%d",
+               ptype);
+      h_grp = H5Gcreate(h_file, partTypeGroupName, H5P_DEFAULT, H5P_DEFAULT,
+                        H5P_DEFAULT);
+      if (h_grp < 0) {
+        error("Error while creating particle group.\n");
+      }
+
+      /* Close particle group */
+      H5Gclose(h_grp);
+    }
 
     /* Close file */
     H5Fclose(h_file);
   }
 
   /* Now loop over ranks and write the data */
-  for (rank = 0; rank < mpi_size; ++rank) {
+  for (int rank = 0; rank < mpi_size; ++rank) {
 
     /* Is it this rank's turn to write ? */
     if (rank == mpi_rank) {
@@ -659,18 +744,57 @@ void write_output_serial(struct engine* e, struct UnitSystem* us, int mpi_rank,
       if (h_file < 0)
         error("Error while opening file '%s' on rank %d.", fileName, mpi_rank);
 
-      /* Open SPH particles group */
-      /* message("Reading particle arrays..."); */
-      h_grp = H5Gopen(h_file, "/PartType0", H5P_DEFAULT);
-      if (h_grp < 0)
-        error("Error while opening particle group on rank %d.\n", mpi_rank);
-
-      /* Write particle fields from the particle structure */
-      hydro_write_particles(h_grp, fileName, xmfFile, N, N_total, mpi_rank,
-                            offset, parts, us);
-
-      /* Close particle group */
-      H5Gclose(h_grp);
+      /* Loop over all particle types */
+      for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ptype++) {
+
+        /* Don't do anything if no particle of this kind */
+        if (N_total[ptype] == 0) continue;
+
+        /* Open the particle group in the file */
+        char partTypeGroupName[PARTICLE_GROUP_BUFFER_SIZE];
+        snprintf(partTypeGroupName, PARTICLE_GROUP_BUFFER_SIZE, "/PartType%d",
+                 ptype);
+        h_grp = H5Gopen(h_file, partTypeGroupName, H5P_DEFAULT);
+        if (h_grp < 0) {
+          error("Error while opening particle group %s.", partTypeGroupName);
+        }
+
+        /* Read particle fields into the particle structure */
+        switch (ptype) {
+
+          case GAS:
+            hydro_write_particles(h_grp, fileName, xmfFile, N[ptype],
+                                  N_total[ptype], mpi_rank, offset[ptype],
+                                  parts, us);
+
+            break;
+
+          case DM:
+            /* Allocate temporary array */
+            if (posix_memalign((void*)&dmparts, gpart_align,
+                               Ndm * sizeof(struct gpart)) != 0)
+              error("Error while allocating temporart memory for DM particles");
+            bzero(dmparts, Ndm * sizeof(struct gpart));
+
+            /* Collect the DM particles from gpart */
+            collect_dm_gparts(gparts, Ntot, dmparts, Ndm);
+
+            /* Write DM particles */
+            darkmatter_write_particles(h_grp, fileName, xmfFile, N[ptype],
+                                       N_total[ptype], mpi_rank, offset[ptype],
+                                       dmparts, us);
+
+            /* Free temporary array */
+            free(dmparts);
+            break;
+
+          default:
+            error("Particle Type %d not yet supported. Aborting", ptype);
+        }
+
+        /* Close particle group */
+        H5Gclose(h_grp);
+      }
 
       /* Close file */
       H5Fclose(h_file);
diff --git a/src/serial_io.h b/src/serial_io.h
index 95f09f5977a97a359e978db7a1b71b02030d6a14..5a34d420cfabd88d4147e3f3630e0efe89951c41 100644
--- a/src/serial_io.h
+++ b/src/serial_io.h
@@ -32,8 +32,9 @@
 #if defined(HAVE_HDF5) && defined(WITH_MPI) && !defined(HAVE_PARALLEL_HDF5)
 
 void read_ic_serial(char* fileName, double dim[3], struct part** parts,
-                    size_t* N, int* periodic, int mpi_rank, int mpi_size,
-                    MPI_Comm comm, MPI_Info info);
+                    struct gpart** gparts, size_t* Ngas, size_t* Ngparts,
+                    int* periodic, int mpi_rank, int mpi_size, MPI_Comm comm,
+                    MPI_Info info);
 
 void write_output_serial(struct engine* e, struct UnitSystem* us, int mpi_rank,
                          int mpi_size, MPI_Comm comm, MPI_Info info);
diff --git a/src/single_io.c b/src/single_io.c
index 59686a68b5d9e5ea41267ba7b3aad9391862fae4..54ace5c4e369cd6b9a02d44f8a8f03ef6cbadd5f 100644
--- a/src/single_io.c
+++ b/src/single_io.c
@@ -39,9 +39,6 @@
 #include "common_io.h"
 #include "error.h"
 
-#define FILENAME_BUFFER_SIZE 150
-#define PARTICLE_GROUP_BUFFER_SIZE 20
-
 /*-----------------------------------------------------------------------------
  * Routines reading an IC file
  *-----------------------------------------------------------------------------*/
@@ -66,14 +63,14 @@
  * Calls #error() if an error occurs.
  */
 void readArrayBackEnd(hid_t grp, char* name, enum DATA_TYPE type, int N,
-                      int dim, char* part_c, enum DATA_IMPORTANCE importance) {
+                      int dim, char* part_c, size_t partSize,
+                      enum DATA_IMPORTANCE importance) {
   hid_t h_data = 0, h_err = 0, h_type = 0;
   htri_t exist = 0;
   void* temp;
   int i = 0;
   const size_t typeSize = sizeOfType(type);
   const size_t copySize = typeSize * dim;
-  const size_t partSize = sizeof(struct part);
   char* temp_c = 0;
 
   /* Check whether the dataspace exists or not */
@@ -158,14 +155,13 @@ void readArrayBackEnd(hid_t grp, char* name, enum DATA_TYPE type, int N,
  */
 void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
                        enum DATA_TYPE type, int N, int dim, char* part_c,
-                       struct UnitSystem* us,
+                       size_t partSize, struct UnitSystem* us,
                        enum UnitConversionFactor convFactor) {
   hid_t h_data = 0, h_err = 0, h_space = 0, h_prop = 0;
   void* temp = 0;
   int i = 0, rank = 0;
   const size_t typeSize = sizeOfType(type);
   const size_t copySize = typeSize * dim;
-  const size_t partSize = sizeof(struct part);
   char* temp_c = 0;
   hsize_t shape[2];
   hsize_t chunk_shape[2];
@@ -204,7 +200,7 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
 
   /* Make sure the chunks are not larger than the dataset */
   if (chunk_shape[0] > N) chunk_shape[0] = N;
-  
+
   /* Change shape of data space */
   h_err = H5Sset_extent_simple(h_space, rank, shape, NULL);
   if (h_err < 0) {
@@ -276,7 +272,7 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
 #define readArray(grp, name, type, N, dim, part, N_total, offset, field, \
                   importance)                                            \
   readArrayBackEnd(grp, name, type, N, dim, (char*)(&(part[0]).field),   \
-                   importance)
+                   sizeof(part[0]), importance)
 
 /**
  * @brief A helper macro to call the readArrayBackEnd function more easily.
@@ -301,7 +297,8 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
 #define writeArray(grp, fileName, xmfFile, name, type, N, dim, part, N_total, \
                    mpi_rank, offset, field, us, convFactor)                   \
   writeArrayBackEnd(grp, fileName, xmfFile, name, type, N, dim,               \
-                    (char*)(&(part[0]).field), us, convFactor)
+                    (char*)(&(part[0]).field), sizeof(part[0]), us,           \
+                    convFactor)
 
 /* Import the right hydro definition */
 #include "hydro_io.h"
@@ -314,9 +311,9 @@ void writeArrayBackEnd(hid_t grp, char* fileName, FILE* xmfFile, char* name,
  * @param fileName The file to read.
  * @param dim (output) The dimension of the volume.
  * @param parts (output) Array of Gas particles.
- * @param gparts (output) Array of DM particles.
+ * @param gparts (output) Array of #gpart particles.
  * @param Ngas (output) number of Gas particles read.
- * @param Ngparts (output) The number of DM particles read.
+ * @param Ngparts (output) The number of #gpart read.
  * @param periodic (output) 1 if the volume is periodic, 0 if not.
  *
  * Opens the HDF5 file fileName and reads the particles contained
@@ -337,6 +334,8 @@ void read_ic_single(char* fileName, double dim[3], struct part** parts,
   double boxSize[3] = {0.0, -1.0, -1.0};
   /* GADGET has 6 particle types. We only keep the type 0 & 1 for now...*/
   int numParticles[NUM_PARTICLE_TYPES] = {0};
+  int numParticles_highWord[NUM_PARTICLE_TYPES] = {0};
+  size_t N[NUM_PARTICLE_TYPES] = {0};
   size_t Ndm;
 
   /* Open file */
@@ -365,9 +364,12 @@ void read_ic_single(char* fileName, double dim[3], struct part** parts,
   /* Read the relevant information and print status */
   readAttribute(h_grp, "BoxSize", DOUBLE, boxSize);
   readAttribute(h_grp, "NumPart_Total", UINT, numParticles);
+  readAttribute(h_grp, "NumPart_Total_HighWord", UINT, numParticles_highWord);
+
+  for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ++ptype)
+    N[ptype] = ((long long)numParticles[ptype]) +
+               ((long long)numParticles_highWord[ptype] << 32);
 
-  *Ngas = numParticles[0];
-  Ndm = numParticles[1];
   dim[0] = boxSize[0];
   dim[1] = (boxSize[1] < 0) ? boxSize[0] : boxSize[1];
   dim[2] = (boxSize[2] < 0) ? boxSize[0] : boxSize[2];
@@ -378,16 +380,16 @@ void read_ic_single(char* fileName, double dim[3], struct part** parts,
   /* Close header */
   H5Gclose(h_grp);
 
-  /* Total number of particles */
-  *Ngparts = *Ngas + Ndm;
-
   /* Allocate memory to store SPH particles */
+  *Ngas = N[0];
   if (posix_memalign((void*)parts, part_align, *Ngas * sizeof(struct part)) !=
       0)
     error("Error while allocating memory for SPH particles");
   bzero(*parts, *Ngas * sizeof(struct part));
 
   /* Allocate memory to store all particles */
+  Ndm = N[1];
+  *Ngparts = N[1] + N[0];
   if (posix_memalign((void*)gparts, gpart_align,
                      *Ngparts * sizeof(struct gpart)) != 0)
     error("Error while allocating memory for gravity particles");
@@ -396,16 +398,14 @@ void read_ic_single(char* fileName, double dim[3], struct part** parts,
   /* message("Allocated %8.2f MB for particles.", *N * sizeof(struct part) /
    * (1024.*1024.)); */
 
-  /* Open SPH particles group */
-  /* message("Reading particle arrays..."); */
-  message("BoxSize = %lf", dim[0]);
-  message("NumPart = [%zd, %zd] Total = %zd", *Ngas, Ndm, *Ngparts);
+  /* message("BoxSize = %lf", dim[0]); */
+  /* message("NumPart = [%zd, %zd] Total = %zd", *Ngas, Ndm, *Ngparts); */
 
   /* Loop over all particle types */
   for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ptype++) {
 
     /* Don't do anything if no particle of this kind */
-    if (numParticles[ptype] == 0) continue;
+    if (N[ptype] == 0) continue;
 
     /* Open the particle group in the file */
     char partTypeGroupName[PARTICLE_GROUP_BUFFER_SIZE];
@@ -476,10 +476,13 @@ void write_output_single(struct engine* e, struct UnitSystem* us) {
   static int outputCount = 0;
 
   /* Number of particles of each type */
-  const size_t Ndm = Ntot - Ngas;
-  int numParticles[NUM_PARTICLE_TYPES] = /* Gadget-2 convention here */
-      {Ngas, Ndm, 0};                    /* Could use size_t instead */
-  int numParticlesHighWord[NUM_PARTICLE_TYPES] = {0};
+  //const size_t Ndm = Ntot - Ngas;
+
+  /* MATTHIEU: Temporary fix to preserve master */
+  const size_t Ndm = Ntot > 0 ? Ntot - Ngas: 0;
+  /* MATTHIEU: End temporary fix */
+
+  long long N_total[NUM_PARTICLE_TYPES] = {Ngas, Ndm, 0};
 
   /* File name */
   char fileName[FILENAME_BUFFER_SIZE];
@@ -521,19 +524,27 @@ void write_output_single(struct engine* e, struct UnitSystem* us) {
 
   /* Print the relevant information and print status */
   writeAttribute(h_grp, "BoxSize", DOUBLE, e->s->dim, 3);
-  writeAttribute(h_grp, "NumPart_ThisFile", UINT, numParticles,
-                 NUM_PARTICLE_TYPES);
   double dblTime = e->time;
   writeAttribute(h_grp, "Time", DOUBLE, &dblTime, 1);
 
   /* GADGET-2 legacy values */
+  /* Number of particles of each type */
+  unsigned int numParticles[NUM_PARTICLE_TYPES] = {0};
+  unsigned int numParticlesHighWord[NUM_PARTICLE_TYPES] = {0};
+  for (int ptype = 0; ptype < NUM_PARTICLE_TYPES; ++ptype) {
+    numParticles[ptype] = (unsigned int)N_total[ptype];
+    numParticlesHighWord[ptype] = (unsigned int)(N_total[ptype] >> 32);
+  }
+  writeAttribute(h_grp, "NumPart_ThisFile", LONGLONG, N_total,
+                 NUM_PARTICLE_TYPES);
   writeAttribute(h_grp, "NumPart_Total", UINT, numParticles,
                  NUM_PARTICLE_TYPES);
   writeAttribute(h_grp, "NumPart_Total_HighWord", UINT, numParticlesHighWord,
                  NUM_PARTICLE_TYPES);
-  double MassTable[NUM_PARTICLE_TYPES] = {0., 0., 0., 0., 0., 0.};
+  double MassTable[NUM_PARTICLE_TYPES] = {0};
   writeAttribute(h_grp, "MassTable", DOUBLE, MassTable, NUM_PARTICLE_TYPES);
-  writeAttribute(h_grp, "Flag_Entropy_ICs", UINT, numParticlesHighWord,
+  unsigned int flagEntropy[NUM_PARTICLE_TYPES] = {0};
+  writeAttribute(h_grp, "Flag_Entropy_ICs", UINT, flagEntropy,
                  NUM_PARTICLE_TYPES);
   writeAttribute(h_grp, "NumFilesPerSnapshot", INT, &numFiles, 1);