Revert Amber-MD#1159

Author: Daniel R. Roe

src/EnergyKernel_NonBond_Simple.h

@@ -12,14 +12,14 @@ template <class REAL> class EnergyKernel_NonBond_Simple {
      /// CONSTRUCTOR - specified cutoff^2
      EnergyKernel_NonBond_Simple(REAL cutIn) : cutoff2_(cutIn) {}
      /// Energy/forces
-     void Calc_F_E(Frame&, int, int, double, double, double, double, double, double, double, double, CharMask const&, double&, double&);
+     void Calc_F_E(Frame&, int, int, double, double, double, double, double, double, double, CharMask const&, double&, double&);
   private:
     REAL cutoff2_; ///< Only calculate interactions within this cutoff (squared)
 };
 
 template<class REAL> 
 void EnergyKernel_NonBond_Simple<REAL>::Calc_F_E(Frame& frameIn, int idx, int jdx,
-                                           double LJA, double LJB, double LJC,
+                                           double LJA, double LJB,
                                            double QFAC, double qi, double qj,
                                            double enbfac, double eelfac,
                                            CharMask const& maskIn,
@@ -35,13 +35,11 @@ void EnergyKernel_NonBond_Simple<REAL>::Calc_F_E(Frame& frameIn, int idx, int jd
     REAL rij = sqrt( rij2 );
     // VDW
     REAL r2    = 1.0 / rij2;
-    REAL r4    = r2 * r2;
-    REAL r6    = r4 * r2;
+    REAL r6    = r2 * r2 * r2;
     REAL r12   = r6 * r6;
     REAL f12   = LJA * r12;  // A/r^12
     REAL f6    = LJB * r6;   // B/r^6
-    REAL f4    = LJC * r4;   // C/r^4
-    REAL e_vdw = f12 - f6 - f4;   // (A/r^12)-(B/r^6)-(C/r^4)
+    REAL e_vdw = f12 - f6;   // (A/r^12)-(B/r^6)
     //mprintf("DBG:\t\t%8i %8i %12.4f\n", e_vdw);
     E_vdw += (e_vdw * enbfac);
     // VDW force
@@ -58,11 +56,7 @@ void EnergyKernel_NonBond_Simple<REAL>::Calc_F_E(Frame& frameIn, int idx, int jd
 //    dfy += ry * felec;
 //    dfz += rz * felec;
     // COMBINED Vdw and Coulomb force 
-    // Force = -dU/dr
-    // U = A/r^12 - B/r^6 - C/r^4
-    // F = 12A/r^13 - 6B/r^7 - 4C/r^5
-    // F_term = F * r = 12A/r^12 - 6B/r^6 - 4C/r^4 = 12f12 - 6f6 - 4f4
-    REAL dfn = ( (((12*f12) - (6*f6) - (4*f4)) * enbfac) + e_elec ) * r2;
+    REAL dfn = ( (((12*f12) - (6*f6)) * enbfac) + e_elec ) * r2;
     REAL dfx = rx * dfn;
     REAL dfy = ry * dfn;
     REAL dfz = rz * dfn;