Add the option to use the Gadget-2 physical constants rather than the more up-to-date default ones.

configure.ac

@@ -2151,6 +2151,17 @@ case "$with_star_formation" in
    ;;
 esac 
 
+AC_ARG_WITH([gadget2-physical-constants],
+    [AS_HELP_STRING([--with-gadget2-physical-constants],
+       [Use the same physical constants (G, Msun, Mpc) as the Gadget-2 code rather than more up-to-date values.]
+    )],
+    [with_gadget2_physical_constants="$withval"],
+    [with_gadget2_physical_constants="no"]
+)
+if test "$with_gadget2_physical_constants" = "yes"; then
+   AC_DEFINE([SWIFT_USE_GADGET2_PHYSICAL_CONSTANTS],1,[Use the same physical constants as Gadget-2])
+fi
+
 #  Gravity multipole order
 AC_ARG_WITH([multipole-order],
    [AS_HELP_STRING([--with-multipole-order=<order>],

src/physical_constants_cgs.h

@@ -40,9 +40,18 @@
  * The primordial Helium fraction is the value obtained by WMAP7.
  */
 
+#ifdef SWIFT_USE_GADGET2_PHYSICAL_CONSTANTS
+
+/*! Newton's gravitation constant [g^-1 cm^3 s^-2] */
+const double const_newton_G_cgs = 6.672e-8;
+
+#else
+
 /*! Newton's gravitation constant [g^-1 cm^3 s^-2] */
 const double const_newton_G_cgs = 6.67408e-8;
 
+#endif
+
 /*! Speed of light in vacuum [cm s^-1] */
 const double const_speed_light_c_cgs = 2.99792458e10;
 
@@ -82,15 +91,33 @@ const double const_year_cgs = 3.15569252e7;
 /*! Astronomical unit [cm] */
 const double const_astronomical_unit_cgs = 1.49597870700e13;
 
+#ifdef SWIFT_USE_GADGET2_PHYSICAL_CONSTANTS
+
+/*! Parsec [cm] */
+const double const_parsec_cgs = 3.085678e18;
+
+#else
+
 /*! Parsec [cm] */
 const double const_parsec_cgs = 3.08567758149e18;
 
+#endif
+
 /*! Light-year [cm] */
 const double const_light_year_cgs = 9.46053e17;
 
+#ifdef SWIFT_USE_GADGET2_PHYSICAL_CONSTANTS
+
+/*! Mass of the Sun [g] */
+const double const_solar_mass_cgs = 1.989e33;
+#error aa
+#else
+
 /*! Mass of the Sun [g] */
 const double const_solar_mass_cgs = 1.98848e33;
 
+#endif
+
 /*! Mass of the Earth [g] */
 const double const_earth_mass_cgs = 5.9724e27;