Throw exception if the user tries to compute forces without setting positions first.

openmmapi/include/openmm/internal/ContextImpl.h

@@ -246,7 +246,7 @@ private:
     std::vector<ForceImpl*> forceImpls;
     std::map<std::string, double> parameters;
     mutable std::vector<std::vector<int> > molecules;
-    bool hasInitializedForces;
+    bool hasInitializedForces, hasSetPositions;
     int lastForceGroups;
     Platform* platform;
     Kernel initializeForcesKernel, kineticEnergyKernel, updateStateDataKernel, applyConstraintsKernel, virtualSitesKernel;

openmmapi/src/ContextImpl.cpp

@@ -48,7 +48,7 @@ using namespace OpenMM;
 using namespace std;
 
 ContextImpl::ContextImpl(Context& owner, System& system, Integrator& integrator, Platform* platform, const map<string, string>& properties) :
-         owner(owner), system(system), integrator(integrator), hasInitializedForces(false), lastForceGroups(-1), platform(platform), platformData(NULL) {
+         owner(owner), system(system), integrator(integrator), hasInitializedForces(false), hasSetPositions(false), lastForceGroups(-1), platform(platform), platformData(NULL) {
     if (system.getNumParticles() == 0)
         throw OpenMMException("Cannot create a Context for a System with no particles");
     
@@ -143,6 +143,7 @@ void ContextImpl::getPositions(std::vector<Vec3>& positions) {
 }
 
 void ContextImpl::setPositions(const std::vector<Vec3>& positions) {
+    hasSetPositions = true;
     updateStateDataKernel.getAs<UpdateStateDataKernel>().setPositions(*this, positions);
     integrator.stateChanged(State::Positions);
 }
@@ -200,6 +201,8 @@ void ContextImpl::computeVirtualSites() {
 }
 
 double ContextImpl::calcForcesAndEnergy(bool includeForces, bool includeEnergy, int groups) {
+    if (!hasSetPositions)
+        throw OpenMMException("Particle positions have not been set");
     lastForceGroups = groups;
     CalcForcesAndEnergyKernel& kernel = initializeForcesKernel.getAs<CalcForcesAndEnergyKernel>();
     double energy = 0.0;