Make use of the Plummer conversion constant for the time-step as well.

src/gravity/Default/gravity.h

@@ -107,8 +107,8 @@ gravity_compute_timestep_self(const struct gpart* const gp,
   const float epsilon = gravity_get_softening(gp, grav_props);
 
   /* Note that 0.66666667 = 2. (from Gadget) / 3. (Plummer softening) */
-  const float dt =
-      sqrtf(0.66666667f * cosmo->a * grav_props->eta * epsilon * ac_inv);
+  const float dt = sqrtf(2. * kernel_gravity_softening_plummer_equivalent_inv *
+                         cosmo->a * grav_props->eta * epsilon * ac_inv);
 
   return dt;
 }

src/kernel_gravity.h

@@ -28,6 +28,7 @@
 
 /*! Conversion factor between Plummer softening and internal softening */
 #define kernel_gravity_softening_plummer_equivalent 3.
+#define kernel_gravity_softening_plummer_equivalent_inv (1. / 3.)
 
 /**
  * @brief Computes the gravity softening function for potential.