Implemented switching function for GayBerneForce

platforms/reference/src/SimTKReference/ReferenceGayBerneForce.cpp

@@ -229,6 +229,15 @@ RealOpenMM ReferenceGayBerneForce::computeOneInteraction(int particle1, int part
     Matrix G12inv = G12.inverse();
     RealOpenMM detG12 = G12.determinant();
     
+    // Compute the switching function.
+
+    RealOpenMM switchValue = 1, switchDeriv = 0;
+    if (useSwitchingFunction && r > switchingDistance) {
+        RealOpenMM t = (r-switchingDistance)/(cutoffDistance-switchingDistance);
+        switchValue = 1+t*t*t*(-10+t*(15-t*6));
+        switchDeriv = t*t*(-30+t*(60-t*30))/(cutoffDistance-switchingDistance);
+    }
+
     // Estimate the distance between the ellipsoids and compute the first terms needed for the energy.
 
     RealOpenMM sigma12 = 1/SQRT(0.5*drUnit.dot(G12inv*drUnit));
@@ -240,6 +249,7 @@ RealOpenMM ReferenceGayBerneForce::computeOneInteraction(int particle1, int part
     RealOpenMM eta = SQRT(2*s[particle1]*s[particle2]/detG12);
     RealOpenMM chi = 2*drUnit.dot(B12inv*drUnit);
     chi *= chi;
+    RealOpenMM energy = u*eta*chi;
     
     // Compute the terms needed for the force.
 
@@ -250,7 +260,7 @@ RealOpenMM ReferenceGayBerneForce::computeOneInteraction(int particle1, int part
     RealOpenMM temp = 0.5*sigma12*sigma12*sigma12*rInv2;
     RealVec dudr = (drUnit + (kappa-drUnit*kappa.dot(drUnit))*temp)*dUSLJdr;
     RealVec dchidr = (iota-drUnit*iota.dot(drUnit))*(-8*rInv2*SQRT(chi));
-    RealVec force = (dchidr*u + dudr*chi)*eta;
+    RealVec force = (dchidr*u + dudr*chi)*(eta*switchValue) - drUnit*(energy*switchDeriv);
     forces[particle1] += force;
     forces[particle2] -= force;
 
@@ -314,8 +324,8 @@ RealOpenMM ReferenceGayBerneForce::computeOneInteraction(int particle1, int part
         RealVec detadq;
         for (int i = 0; i < 3; i++)
             detadq += RealVec(a[i][0], a[i][1], a[i][2]).cross(RealVec(d[i][0], d[i][1], d[i][2]));
-        RealVec torque = dchidq*(u*eta) + detadq*(u*chi) + dudq*(eta*chi);
+        RealVec torque = (dchidq*(u*eta) + detadq*(u*chi) + dudq*(eta*chi))*switchValue;
         torques[particle] -= torque;
     }
-    return u*eta*chi;
+    return switchValue*energy;
 }

tests/TestGayBerneForce.h

@@ -146,6 +146,8 @@ void testEnergyConservation() {
     system.addForce(gb);
     gb->setNonbondedMethod(GayBerneForce::CutoffPeriodic);
     gb->setCutoffDistance(1.0);
+    gb->setUseSwitchingFunction(true);
+    gb->setSwitchingDistance(0.9);
     vector<Vec3> positions;
     for (int x = 0; x < 3; x++) {
         for (int y = 0; y < 3; y++) {
@@ -178,7 +180,7 @@ void testEnergyConservation() {
         if (i == 0)
             initialEnergy = energy;
         else
-            ASSERT_EQUAL_TOL(initialEnergy, energy, 1e-3);
+            ASSERT_EQUAL_TOL(initialEnergy, energy, 1e-4);
     }
 }
 

wrappers/python/src/swig_doxygen/swigInputConfig.py

@@ -412,6 +412,8 @@ UNITS = {
 ("PeriodicTorsionForce", "getTorsionParameters")
  : (None, (None, None, None, None,
            None, 'unit.radian', 'unit.kilojoule_per_mole')),
+("GayBerneForce", "getParticleParameters")
+ : (None, ('unit.nanometer', 'unit.kilojoule_per_mole', None, None, 'unit.nanometer', 'unit.nanometer', 'unit.nanometer', None, None, None)),
 ("Platform", "getDefaultPluginsDirectory") : (None, ()),
 ("Platform", "getPropertyDefaultValue") : (None, ()),
 ("Platform", "getPropertyNames") : (None, ()),