added brownian dynamics integrator fix bd/sphere.

src/random_mars.cpp

@@ -94,6 +94,22 @@ double RanMars::uniform()
   return uni;
 }
 
+/* ----------------------------------------------------------------------
+   uniform RN shifted to be symmetric about zero (for fix bd/sphere).
+------------------------------------------------------------------------- */
+double RanMars::uniform_middle()
+{
+  return uniform()-0.5;
+}
+
+/* ----------------------------------------------------------------------
+   Return 0 (for fix/bd/sphere).
+------------------------------------------------------------------------- */
+double RanMars::zero_rng()
+{
+  return 0.0;
+}
+
 /* ----------------------------------------------------------------------
    gaussian RN
 ------------------------------------------------------------------------- */