RPMDIntegrator throws an exception if the System includes virtual sites

plugins/rpmd/openmmapi/src/RPMDIntegrator.cpp

@@ -54,8 +54,12 @@ RPMDIntegrator::RPMDIntegrator(int numCopies, double temperature, double frictio
 void RPMDIntegrator::initialize(ContextImpl& contextRef) {
     if (owner != NULL && &contextRef.getOwner() != owner)
         throw OpenMMException("This Integrator is already bound to a context");
-    if (contextRef.getSystem().getNumConstraints() > 0)
+    const System& system = contextRef.getSystem();
+    if (system.getNumConstraints() > 0)
         throw OpenMMException("RPMDIntegrator cannot be used with Systems that include constraints");
+    for (int i = 0; i < system.getNumParticles(); i++)
+        if (system.isVirtualSite(i))
+            throw OpenMMException("RPMDIntegrator cannot be used with Systems that include virtual sites");
     context = &contextRef;
     owner = &contextRef.getOwner();
     kernel = context->getPlatform().createKernel(IntegrateRPMDStepKernel::Name(), contextRef);