Bug fix to custom integrator

openmmapi/include/openmm/internal/ContextImpl.h

@@ -197,8 +197,11 @@ public:
     double calcForcesAndEnergy(bool includeForces, bool includeEnergy, int groups=0xFFFFFFFF);
     /**
      * Get the set of force group flags that were passed to the most recent call to calcForcesAndEnergy().
+     * 
+     * Note that this returns a reference, so it's possible to modify it.  Be very very cautious about
+     * doing that!  Only do it if you're also modifying forces stored inside the context.
      */
-    int getLastForceGroups() const;
+    int& getLastForceGroups();
     /**
      * Calculate the kinetic energy of the system (in kJ/mol).
      */

openmmapi/src/ContextImpl.cpp

@@ -301,7 +301,7 @@ double ContextImpl::calcForcesAndEnergy(bool includeForces, bool includeEnergy,
     }
 }
 
-int ContextImpl::getLastForceGroups() const {
+int& ContextImpl::getLastForceGroups() {
     return lastForceGroups;
 }
 

openmmapi/src/CustomIntegratorUtilities.cpp

@@ -111,7 +111,8 @@ void CustomIntegratorUtilities::analyzeComputations(const ContextImpl& context,
         for (auto& param : force->getDefaultParameters())
             affectsForce.insert(param.first);
     for (int i = 0; i < numSteps; i++)
-        invalidatesForces[i] = (stepType[i] == CustomIntegrator::ConstrainPositions || affectsForce.find(stepVariable[i]) != affectsForce.end());
+        invalidatesForces[i] = (stepType[i] == CustomIntegrator::ConstrainPositions || stepType[i] == CustomIntegrator::UpdateContextState ||
+                affectsForce.find(stepVariable[i]) != affectsForce.end());
 
     // Make a list of which steps require valid forces or energy to be known.
 

platforms/cuda/src/CudaKernels.cpp

@@ -7604,12 +7604,14 @@ void CudaIntegrateCustomStepKernel::execute(ContextImpl& context, CustomIntegrat
         int nextStep = step+1;
         int lastForceGroups = context.getLastForceGroups();
         if ((needsForces[step] || needsEnergy[step]) && (!forcesAreValid || lastForceGroups != forceGroupFlags[step])) {
-            if (forcesAreValid && savedForces.find(lastForceGroups) != savedForces.end()) {
-                // The forces are still valid.  We just need a different force group right now.  Save the old
-                // forces in case we need them again.
-                
-                cu.getForce().copyTo(*savedForces[lastForceGroups]);
-                validSavedForces.insert(lastForceGroups);
+            if (forcesAreValid) {
+                if (savedForces.find(lastForceGroups) != savedForces.end() && validSavedForces.find(lastForceGroups) == validSavedForces.end()) {
+                    // The forces are still valid.  We just need a different force group right now.  Save the old
+                    // forces in case we need them again.
+
+                    cu.getForce().copyTo(*savedForces[lastForceGroups]);
+                    validSavedForces.insert(lastForceGroups);
+                }
             }
             else
                 validSavedForces.clear();
@@ -7623,6 +7625,7 @@ void CudaIntegrateCustomStepKernel::execute(ContextImpl& context, CustomIntegrat
                 // We can just restore the forces we saved earlier.
                 
                 savedForces[forceGroupFlags[step]]->copyTo(cu.getForce());
+                context.getLastForceGroups() = forceGroupFlags[step];
             }
             else {
                 recordChangedParameters(context);

platforms/opencl/src/OpenCLKernels.cpp

@@ -7944,12 +7944,14 @@ void OpenCLIntegrateCustomStepKernel::execute(ContextImpl& context, CustomIntegr
         int nextStep = step+1;
         int lastForceGroups = context.getLastForceGroups();
         if ((needsForces[step] || needsEnergy[step]) && (!forcesAreValid || lastForceGroups != forceGroupFlags[step])) {
-            if (forcesAreValid && savedForces.find(lastForceGroups) != savedForces.end()) {
-                // The forces are still valid.  We just need a different force group right now.  Save the old
-                // forces in case we need them again.
-                
-                cl.getForce().copyTo(*savedForces[lastForceGroups]);
-                validSavedForces.insert(lastForceGroups);
+            if (forcesAreValid) {
+                if (savedForces.find(lastForceGroups) != savedForces.end() && validSavedForces.find(lastForceGroups) == validSavedForces.end()) {
+                    // The forces are still valid.  We just need a different force group right now.  Save the old
+                    // forces in case we need them again.
+
+                    cl.getForce().copyTo(*savedForces[lastForceGroups]);
+                    validSavedForces.insert(lastForceGroups);
+                }
             }
             else
                 validSavedForces.clear();
@@ -7963,6 +7965,7 @@ void OpenCLIntegrateCustomStepKernel::execute(ContextImpl& context, CustomIntegr
                 // We can just restore the forces we saved earlier.
                 
                 savedForces[forceGroupFlags[step]]->copyTo(cl.getForce());
+                context.getLastForceGroups() = forceGroupFlags[step];
             }
             else {
                 recordChangedParameters(context);

tests/TestCustomIntegrator.h

@@ -37,6 +37,7 @@
 #include "openmm/AndersenThermostat.h"
 #include "openmm/CustomAngleForce.h"
 #include "openmm/CustomBondForce.h"
+#include "openmm/CustomExternalForce.h"
 #include "openmm/CustomIntegrator.h"
 #include "openmm/HarmonicBondForce.h"
 #include "openmm/NonbondedForce.h"
@@ -921,6 +922,52 @@ void testTabulatedFunction() {
     ASSERT_EQUAL_VEC(Vec3(12.0, 13.0, 14.0), values[0], 1e-5);
 }
 
+/**
+ * Test an integrator that alternates repeatedly between force groups.
+ */
+void testAlternatingGroups() {
+    System system;
+    system.addParticle(1.0);
+    CustomExternalForce* force1 = new CustomExternalForce("-0.5*x");
+    force1->addParticle(0);
+    system.addForce(force1);
+    CustomExternalForce* force2 = new CustomExternalForce("-0.8*y");
+    force2->addParticle(0);
+    force2->setForceGroup(1);
+    system.addForce(force2);
+    CustomIntegrator integrator(0.5);
+    integrator.addGlobalVariable("savede1", 0.0);
+    integrator.addGlobalVariable("savede2", 0.0);
+    integrator.addGlobalVariable("savede3", 0.0);
+    integrator.addGlobalVariable("savede4", 0.0);
+    integrator.addPerDofVariable("savedf1", 0.0);
+    integrator.addPerDofVariable("savedf2", 0.0);
+    integrator.addPerDofVariable("savedf3", 0.0);
+    integrator.addPerDofVariable("savedf4", 0.0);
+    integrator.addComputeGlobal("savede1", "energy0");
+    integrator.addComputeGlobal("savede2", "energy1");
+    integrator.addComputePerDof("savedf1", "f0");
+    integrator.addComputePerDof("savedf2", "f1");
+    integrator.addComputeGlobal("savede3", "energy0");
+    integrator.addComputeGlobal("savede4", "energy1");
+    integrator.addComputePerDof("savedf3", "f0");
+    integrator.addComputePerDof("savedf4", "f1");
+    Context context(system, integrator, platform);
+    vector<Vec3> positions(1);
+    positions[0] = Vec3(1, 2, 3);
+    context.setPositions(positions);
+    integrator.step(1);
+    vector<Vec3> f;
+    for (int i = 0; i < 2; i++) {
+        ASSERT_EQUAL_TOL(-0.5*1, integrator.getGlobalVariable(2*i), 1e-5);
+        ASSERT_EQUAL_TOL(-0.8*2, integrator.getGlobalVariable(2*i+1), 1e-5);
+        integrator.getPerDofVariable(2*i, f);
+        ASSERT_EQUAL_VEC(Vec3(0.5, 0, 0), f[0], 1e-5);
+        integrator.getPerDofVariable(2*i+1, f);
+        ASSERT_EQUAL_VEC(Vec3(0, 0.8, 0), f[0], 1e-5);
+    }
+}
+
 void runPlatformTests();
 
 int main(int argc, char* argv[]) {
@@ -944,6 +991,7 @@ int main(int argc, char* argv[]) {
         testEnergyParameterDerivatives();
         testChangeDT();
         testTabulatedFunction();
+        testAlternatingGroups();
         runPlatformTests();
     }
     catch(const exception& e) {