States can save integrator parameters

olla/include/openmm/kernels.h

@@ -1136,6 +1136,22 @@ public:
      * Load the chain states from a checkpoint.
      */
     virtual void loadCheckpoint(ContextImpl& context, std::istream& stream) = 0;
+    /**
+     * Get the internal states of all chains.
+     * 
+     * @param context       the context for which to get the states
+     * @param positions     element [i][j] contains the position of bead j for chain i
+     * @param velocities    element [i][j] contains the velocity of bead j for chain i
+     */
+    virtual void getChainStates(ContextImpl& context, std::vector<std::vector<double> >& positions, std::vector<std::vector<double> >& velocities) const = 0;
+    /**
+     * Set the internal states of all chains.
+     * 
+     * @param context       the context for which to get the states
+     * @param positions     element [i][j] contains the position of bead j for chain i
+     * @param velocities    element [i][j] contains the velocity of bead j for chain i
+     */
+    virtual void setChainStates(ContextImpl& context, const std::vector<std::vector<double> >& positions, const std::vector<std::vector<double> >& velocities) = 0;
 };
 
 /**

openmmapi/include/openmm/CompoundIntegrator.h

@@ -204,6 +204,19 @@ protected:
      * data it wrote in createCheckpoint() and update its internal state accordingly.
      */
     void loadCheckpoint(std::istream& stream);
+    /**
+     * This is called while creating a State.  The Integrator should store the values
+     * of all time-varying parameters into the SerializationNode so they can be saved
+     * as part of the state.
+     */
+    void serializeParameters(SerializationNode& node) const;
+    /**
+     * This is called when loading a previously saved State.  The Integrator should
+     * load the values of all time-varying parameters from the SerializationNode.  If
+     * the node contains parameters that are not defined for this Integrator, it should
+     * throw an exception.
+     */
+    void deserializeParameters(const SerializationNode& node);
 private:
     int currentIntegrator;
     std::vector<Integrator*> integrators;

openmmapi/include/openmm/CustomIntegrator.h

@@ -678,6 +678,19 @@ protected:
      * data it wrote in createCheckpoint() and update its internal state accordingly.
      */
     void loadCheckpoint(std::istream& stream);
+    /**
+     * This is called while creating a State.  The Integrator should store the values
+     * of all time-varying parameters into the SerializationNode so they can be saved
+     * as part of the state.
+     */
+    void serializeParameters(SerializationNode& node) const;
+    /**
+     * This is called when loading a previously saved State.  The Integrator should
+     * load the values of all time-varying parameters from the SerializationNode.  If
+     * the node contains parameters that are not defined for this Integrator, it should
+     * throw an exception.
+     */
+    void deserializeParameters(const SerializationNode& node);
 private:
     class ComputationInfo;
     class FunctionInfo;

openmmapi/include/openmm/Integrator.h

@@ -34,6 +34,7 @@
 
 #include "State.h"
 #include "Vec3.h"
+#include "openmm/serialization/SerializationNode.h"
 #include <iosfwd>
 #include <map>
 #include <vector>
@@ -176,6 +177,21 @@ protected:
      */
     virtual void loadCheckpoint(std::istream& stream) {
     }
+    /**
+     * This is called while creating a State.  The Integrator should store the values
+     * of all time-varying parameters into the SerializationNode so they can be saved
+     * as part of the state.
+     */
+    virtual void serializeParameters(SerializationNode& node) const {
+    }
+    /**
+     * This is called when loading a previously saved State.  The Integrator should
+     * load the values of all time-varying parameters from the SerializationNode.  If
+     * the node contains parameters that are not defined for this Integrator, it should
+     * throw an exception.
+     */
+    virtual void deserializeParameters(const SerializationNode& node) {
+    }
 private:
     double stepSize, constraintTol;
     int forceGroups;

openmmapi/include/openmm/NoseHooverIntegrator.h

@@ -9,9 +9,9 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2019 Stanford University and the Authors.           *
+ * Portions copyright (c) 2019-2020 Stanford University and the Authors.      *
  * Authors: Andreas Krämer and Andrew C. Simmonett                            *
- * Contributors:                                                              *
+ * Contributors: Peter Eastman                                                *
  *                                                                            *
  * Permission is hereby granted, free of charge, to any person obtaining a    *
  * copy of this software and associated documentation files (the "Software"), *
@@ -270,6 +270,19 @@ protected:
      * data it wrote in createCheckpoint() and update its internal state accordingly.
      */
     void loadCheckpoint(std::istream& stream);
+    /**
+     * This is called while creating a State.  The Integrator should store the values
+     * of all time-varying parameters into the SerializationNode so they can be saved
+     * as part of the state.
+     */
+    void serializeParameters(SerializationNode& node) const;
+    /**
+     * This is called when loading a previously saved State.  The Integrator should
+     * load the values of all time-varying parameters from the SerializationNode.  If
+     * the node contains parameters that are not defined for this Integrator, it should
+     * throw an exception.
+     */
+    void deserializeParameters(const SerializationNode& node);
 
     std::vector<NoseHooverChain> noseHooverChains;
     std::vector<int> allAtoms;

openmmapi/include/openmm/State.h

@@ -9,7 +9,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2008-2016 Stanford University and the Authors.      *
+ * Portions copyright (c) 2008-2020 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -33,6 +33,7 @@
  * -------------------------------------------------------------------------- */
 
 #include "Vec3.h"
+#include "openmm/serialization/SerializationNode.h"
 #include <map>
 #include <string>
 #include <vector>
@@ -58,7 +59,7 @@ public:
      * This is an enumeration of the types of data which may be stored in a State.  When you create
      * a State, use these values to specify which data types it should contain.
      */
-    enum DataType {Positions=1, Velocities=2, Forces=4, Energy=8, Parameters=16, ParameterDerivatives=32};
+    enum DataType {Positions=1, Velocities=2, Forces=4, Energy=8, Parameters=16, ParameterDerivatives=32, IntegratorParameters=64};
     /**
      * Construct an empty State containing no data.  This exists so State objects can be used in STL containers.
      */
@@ -124,6 +125,8 @@ public:
      */
     int getDataTypes() const;
 private:
+    friend class Context;
+    friend class StateProxy;
     State(double time);
     void setPositions(const std::vector<Vec3>& pos);
     void setVelocities(const std::vector<Vec3>& vel);
@@ -132,6 +135,8 @@ private:
     void setEnergyParameterDerivatives(const std::map<std::string, double>& derivs);
     void setEnergy(double ke, double pe);
     void setPeriodicBoxVectors(const Vec3& a, const Vec3& b, const Vec3& c);
+    SerializationNode& updateIntegratorParameters();
+    const SerializationNode& getIntegratorParameters() const;
     int types;
     double time, ke, pe;
     std::vector<Vec3> positions;
@@ -139,6 +144,7 @@ private:
     std::vector<Vec3> forces;
     Vec3 periodicBoxVectors[3];
     std::map<std::string, double> parameters, energyParameterDerivatives;
+    SerializationNode integratorParameters;
 };
 
 /**
@@ -157,6 +163,7 @@ public:
     void setEnergyParameterDerivatives(const std::map<std::string, double>& params);
     void setEnergy(double ke, double pe);
     void setPeriodicBoxVectors(const Vec3& a, const Vec3& b, const Vec3& c);
+    SerializationNode& updateIntegratorParameters();
 private:
     State state;
 };

openmmapi/src/CompoundIntegrator.cpp

@@ -140,3 +140,22 @@ void CompoundIntegrator::loadCheckpoint(std::istream& stream) {
     for (int i = 0; i < integrators.size(); i++)
         integrators[i]->loadCheckpoint(stream);
 }
+
+void CompoundIntegrator::serializeParameters(SerializationNode& node) const {
+    node.setIntProperty("version", 1);
+    node.setIntProperty("currentIntegrator", currentIntegrator);
+    for (int i = 0; i < getNumIntegrators(); i++) {
+        SerializationNode& child = node.createChildNode("IntegratorParameters");
+        integrators[i]->serializeParameters(child);
+    }
+}
+
+void CompoundIntegrator::deserializeParameters(const SerializationNode& node) {
+    if (node.getIntProperty("version") != 1)
+        throw OpenMMException("Unsupported version number");
+    if (node.getChildren().size() != getNumIntegrators())
+        throw OpenMMException("State has wrong number of integrators for CompoundIntegrator");
+    setCurrentIntegrator(node.getIntProperty("currentIntegrator"));
+    for (int i = 0; i < node.getChildren().size(); i++)
+        integrators[i]->deserializeParameters(node.getChildren()[i]);
+}

openmmapi/src/Context.cpp

@@ -147,6 +147,9 @@ State Context::getState(int types, bool enforcePeriodicBox, int groups) const {
         impl->getVelocities(velocities);
         builder.setVelocities(velocities);
     }
+    if (types&State::IntegratorParameters) {
+        getIntegrator().serializeParameters(builder.updateIntegratorParameters());
+    }
     return builder.getState();
 }
 
@@ -162,6 +165,8 @@ void Context::setState(const State& state) {
     if ((state.getDataTypes()&State::Parameters) != 0)
         for (auto& param : state.getParameters())
             setParameter(param.first, param.second);
+    if ((state.getDataTypes()&State::IntegratorParameters) != 0)
+        getIntegrator().deserializeParameters(state.getIntegratorParameters());
 }
 
 void Context::setTime(double time) {

openmmapi/src/CustomIntegrator.cpp

@@ -6,7 +6,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2011-2019 Stanford University and the Authors.      *
+ * Portions copyright (c) 2011-2020 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -348,3 +348,33 @@ void CustomIntegrator::setKineticEnergyExpression(const string& expression) {
     Lepton::CompiledExpression expr = Lepton::Parser::parse(kineticEnergy).createCompiledExpression();
     keNeedsForce = (expr.getVariables().find("f") != expr.getVariables().end());
 }
+
+void CustomIntegrator::serializeParameters(SerializationNode& node) const {
+    node.setIntProperty("version", 1);
+    SerializationNode& globalVariablesNode = node.createChildNode("GlobalVariables");
+    for (int i = 0; i < getNumGlobalVariables(); i++)
+        globalVariablesNode.setDoubleProperty(getGlobalVariableName(i), getGlobalVariable(i));
+    SerializationNode& perDofVariablesNode = node.createChildNode("PerDofVariables");
+    for (int i = 0; i < getNumPerDofVariables(); i++) {
+        SerializationNode& perDofValuesNode = perDofVariablesNode.createChildNode(getPerDofVariableName(i));
+        vector<Vec3> perDofValues;
+        getPerDofVariable(i, perDofValues);
+        for (int j = 0; j < perDofValues.size(); j++)
+            perDofValuesNode.createChildNode("Value").setDoubleProperty("x",perDofValues[j][0]).setDoubleProperty("y",perDofValues[j][1]).setDoubleProperty("z",perDofValues[j][2]);
+    }
+}
+
+void CustomIntegrator::deserializeParameters(const SerializationNode& node) {
+    if (node.getIntProperty("version") != 1)
+        throw OpenMMException("Unsupported version number");
+    const SerializationNode& globalVariablesNode = node.getChildNode("GlobalVariables");
+    for (auto& prop : globalVariablesNode.getProperties())
+        setGlobalVariableByName(prop.first, globalVariablesNode.getDoubleProperty(prop.first));
+    const SerializationNode& perDofVariablesNode = node.getChildNode("PerDofVariables");
+    for (auto& var : perDofVariablesNode.getChildren()) {
+        vector<Vec3> perDofValues;
+        for (auto& child : var.getChildren())
+            perDofValues.push_back(Vec3(child.getDoubleProperty("x"), child.getDoubleProperty("y"), child.getDoubleProperty("z")));
+        setPerDofVariableByName(var.getName(), perDofValues);
+    }
+}

openmmapi/src/NoseHooverIntegrator.cpp

@@ -8,7 +8,7 @@
  *                                                                            *
  * Portions copyright (c) 2019-2020 Stanford University and the Authors.      *
  * Authors: Andreas Krämer and Andrew C. Simmonett                            *
- * Contributors:                                                              *
+ * Contributors: Peter Eastman                                                *
  *                                                                            *
  * Permission is hereby granted, free of charge, to any person obtaining a    *
  * copy of this software and associated documentation files (the "Software"), *
@@ -349,3 +349,29 @@ void NoseHooverIntegrator::createCheckpoint(std::ostream& stream) const {
 void NoseHooverIntegrator::loadCheckpoint(std::istream& stream) {
     kernel.getAs<IntegrateNoseHooverStepKernel>().loadCheckpoint(*context, stream);
 }
+
+void NoseHooverIntegrator::serializeParameters(SerializationNode& node) const {
+    node.setIntProperty("version", 1);
+    vector<vector<double> > positions, velocities;
+    kernel.getAs<IntegrateNoseHooverStepKernel>().getChainStates(*context, positions, velocities);
+    for (int i = 0; i < positions.size(); i++) {
+        SerializationNode& chain = node.createChildNode("Chain");
+        for (int j = 0; j < positions[i].size(); j++)
+            chain.createChildNode("Bead").setDoubleProperty("position", positions[i][j]).setDoubleProperty("velocity", velocities[i][j]);
+    }
+}
+
+void NoseHooverIntegrator::deserializeParameters(const SerializationNode& node) {
+    if (node.getIntProperty("version") != 1)
+        throw OpenMMException("Unsupported version number");
+    int numChains = node.getChildren().size();
+    vector<vector<double> > positions(numChains), velocities(numChains);
+    for (int i = 0; i < numChains; i++) {
+        auto& chain = node.getChildren()[i];
+        for (auto& bead : chain.getChildren()) {
+            positions[i].push_back(bead.getDoubleProperty("position"));
+            velocities[i].push_back(bead.getDoubleProperty("velocity"));
+        }
+    }
+    kernel.getAs<IntegrateNoseHooverStepKernel>().setChainStates(*context, positions, velocities);
+}

openmmapi/src/State.cpp

@@ -81,6 +81,16 @@ const map<string, double>& State::getEnergyParameterDerivatives() const {
         throw OpenMMException("Invoked getEnergyParameterDerivatives() on a State which does not contain parameter derivatives.");
     return energyParameterDerivatives;
 }
+const SerializationNode& State::getIntegratorParameters() const {
+    if ((types&IntegratorParameters) == 0)
+        throw OpenMMException("Invoked getPIntegratorarameters() on a State which does not contain integrator parameters.");
+    return integratorParameters;
+}
+SerializationNode& State::updateIntegratorParameters() {
+    types |= IntegratorParameters;
+    integratorParameters.setName("IntegratorParameters");
+    return integratorParameters;
+}
 int State::getDataTypes() const {
     return types;
 }
@@ -159,3 +169,7 @@ void State::StateBuilder::setEnergy(double ke, double pe) {
 void State::StateBuilder::setPeriodicBoxVectors(const Vec3& a, const Vec3& b, const Vec3& c) {
     state.setPeriodicBoxVectors(a, b, c);
 }
+
+SerializationNode& State::StateBuilder::updateIntegratorParameters() {
+    return state.updateIntegratorParameters();
+}

serialization/src/StateProxy.cpp

@@ -6,7 +6,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2010-2015 Stanford University and the Authors.      *
+ * Portions copyright (c) 2010-2020 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -89,6 +89,9 @@ void StateProxy::serialize(const void* object, SerializationNode& node) const {
             forcesNode.createChildNode("Force").setDoubleProperty("x", stateForces[i][0]).setDoubleProperty("y", stateForces[i][1]).setDoubleProperty("z", stateForces[i][2]);
         }
     }
+    if ((s.getDataTypes()&State::IntegratorParameters) != 0) {
+        node.getChildren().push_back(s.getIntegratorParameters());
+    }
 }
 
 void* StateProxy::deserialize(const SerializationNode& node) const {
@@ -138,6 +141,9 @@ void* StateProxy::deserialize(const SerializationNode& node) const {
             builder.setForces(outForces);
             arraySizes.push_back(outForces.size());
         }
+        else if (child.getName() == "IntegratorParameters") {
+            builder.updateIntegratorParameters() = child;
+        }
     }
     for (int i = 1; i < arraySizes.size(); i++) {
         if (arraySizes[i] != arraySizes[i-1]) {

serialization/tests/TestSerializeState.cpp

@@ -34,6 +34,7 @@
 #include "openmm/NonbondedForce.h"
 #include "openmm/System.h"
 #include "openmm/Context.h"
+#include "openmm/CustomIntegrator.h"
 #include "openmm/LangevinIntegrator.h"
 #include "openmm/AndersenThermostat.h"
 #include "openmm/MonteCarloBarostat.h"
@@ -183,9 +184,42 @@ void testSerialization() {
     }
 }
 
+void testIntegratorParameters() {
+    // Create a Context involving integrator parameters.
+    
+    System system;
+    system.addParticle(1.0);
+    CustomIntegrator integrator(0.001);
+    integrator.addGlobalVariable("a", 1.0);
+    integrator.addPerDofVariable("b", 2.0);
+    Context context(system, integrator);
+    integrator.setGlobalVariable(0, 3.0);
+    integrator.setPerDofVariable(0, {Vec3(1.0, 2.0, 3.0)});
+    
+    // Create a State, then serialize and deserialize it.
+    
+    State s1 = context.getState(State::IntegratorParameters);
+    stringstream buffer;
+    XmlSerializer::serialize<State>(&s1, "State", buffer);
+    State* copy = XmlSerializer::deserialize<State>(buffer);
+    State& s2 = *copy;
+
+    // Set the State on a new Context and make sure all the integrator parameters
+    // survived the serialization and deserialization.
+    
+    CustomIntegrator* integrator2 = XmlSerializer::clone(integrator);
+    Context context2(system, *integrator2);
+    context2.setState(s2);
+    ASSERT_EQUAL(3.0, integrator2->getGlobalVariable(0));
+    vector<Vec3> values;
+    integrator2->getPerDofVariable(0, values);
+    ASSERT_EQUAL_VEC(Vec3(1.0, 2.0, 3.0), values[0], 1e-6);
+}
+
 int main() {
     try {
         testSerialization();
+        testIntegratorParameters();
     }
     catch(const exception& e) {
         cout << "exception: " << e.what() << endl;  

tests/TestCompoundIntegrator.h

@@ -239,6 +239,35 @@ void testCheckpoint() {
     ASSERT_EQUAL_VEC(b1[0], b3[0], 1e-6);
 }
 
+void testSaveParameters() {
+    // Test that integrator variables get loaded correctly from States.
+    System system;
+    system.addParticle(1.0);
+    CustomIntegrator* custom = new CustomIntegrator(0.001);
+    custom->addGlobalVariable("a", 1.0);
+    custom->addPerDofVariable("b", 2.0);
+    CompoundIntegrator integrator;
+    integrator.addIntegrator(custom);
+    integrator.addIntegrator(new VerletIntegrator(0.005));
+    Context context(system, integrator, platform);
+    vector<Vec3> positions(1, Vec3());
+    context.setPositions(positions);
+    custom->setGlobalVariable(0, 5.0);
+    vector<Vec3> b1(1, Vec3(1, 2, 3));
+    custom->setPerDofVariable(0, b1);
+    State savedState = context.getState(State::IntegratorParameters); 
+    custom->setGlobalVariable(0, 10.0);
+    vector<Vec3> b2(1, Vec3(4, 5, 6));
+    custom->setPerDofVariable(0, b2);
+    integrator.setCurrentIntegrator(1);
+    context.setState(savedState);
+    ASSERT_EQUAL(0, integrator.getCurrentIntegrator());
+    ASSERT_EQUAL(5.0, custom->getGlobalVariable(0));
+    vector<Vec3> b3;
+    custom->getPerDofVariable(0, b3);
+    ASSERT_EQUAL_VEC(b1[0], b3[0], 1e-6);
+}
+
 void runPlatformTests();
 
 int main(int argc, char* argv[]) {
@@ -248,6 +277,7 @@ int main(int argc, char* argv[]) {
         testChangingParameters();
         testDifferentStepSizes();
         testCheckpoint();
+        testSaveParameters();
         runPlatformTests();
     }
     catch(const exception& e) {

tests/TestCustomIntegrator.h

@@ -1187,6 +1187,30 @@ void testCheckpoint() {
     ASSERT_EQUAL_VEC(b1[0], b3[0], 1e-6);
 }
 
+void testSaveParameters() {
+    // Test that integrator variables get loaded correctly from States.
+    System system;
+    system.addParticle(1.0);
+    CustomIntegrator integrator(0.001);
+    integrator.addGlobalVariable("a", 1.0);
+    integrator.addPerDofVariable("b", 2.0);
+    Context context(system, integrator, platform);
+    vector<Vec3> positions(1, Vec3());
+    context.setPositions(positions);
+    integrator.setGlobalVariable(0, 5.0);
+    vector<Vec3> b1(1, Vec3(1, 2, 3));
+    integrator.setPerDofVariable(0, b1);
+    State savedState = context.getState(State::IntegratorParameters); 
+    integrator.setGlobalVariable(0, 10.0);
+    vector<Vec3> b2(1, Vec3(4, 5, 6));
+    integrator.setPerDofVariable(0, b2);
+    context.setState(savedState);
+    ASSERT_EQUAL(5.0, integrator.getGlobalVariable(0));
+    vector<Vec3> b3;
+    integrator.getPerDofVariable(0, b3);
+    ASSERT_EQUAL_VEC(b1[0], b3[0], 1e-6);
+}
+
 void runPlatformTests();
 
 int main(int argc, char* argv[]) {
@@ -1216,6 +1240,7 @@ int main(int argc, char* argv[]) {
         testRecordEnergy();
         testInitialTemperature();
         testCheckpoint();
+        testSaveParameters();
         runPlatformTests();
     }
     catch(const exception& e) {

tests/TestNoseHooverIntegrator.h

@@ -499,6 +499,57 @@ void testCheckpoints() {
     ASSERT_EQUAL_VEC(state1.getVelocities()[1], state2.getVelocities()[1], 1e-6); 
 }
 
+void testSaveParameters() {
+    // Create a system with Drude-like particles to be thermostated as a pair, as well as another
+    // particle to be thermostated independently, to test all integrator features.
+    double timeStep = 0.001;
+    NoseHooverIntegrator integrator(timeStep), newIntegrator(timeStep);
+    System system;
+    double mass = 1;
+    system.addParticle(8*mass);
+    system.addParticle(mass);
+    system.addParticle(5*mass);
+    HarmonicBondForce* force = new HarmonicBondForce();
+    force->addBond(0, 1, 0.1, 50.0);
+    force->addBond(0, 2, 0.1, 50.0);
+    system.addForce(force);
+    double kineticEnergy = 1e6;
+    double temperature=300, collisionFrequency=1, chainLength=3, numMTS=3, numYS=3;
+    chainLength = 10;
+    integrator.addSubsystemThermostat(std::vector<int>{2}, std::vector<std::pair<int,int>>{{0,1}},  temperature, collisionFrequency, temperature, collisionFrequency,
+                                      chainLength, numMTS, numYS);
+    newIntegrator.addSubsystemThermostat(std::vector<int>{2}, std::vector<std::pair<int,int>>{{0,1}},  temperature, collisionFrequency, temperature, collisionFrequency,
+                                      chainLength, numMTS, numYS);
+    Context context(system, integrator, platform);
+    Context newContext(system, newIntegrator, platform);
+    std::vector<Vec3> positions(3);
+    std::vector<Vec3> velocities(3);
+    positions[1] = {0.1, 0.0, 0.0};
+    velocities[1] = {0.1,0.2,-0.2};
+    positions[2] = {-0.1, 0.001, 0.001};
+    velocities[2] = {-0.1,0.2,-0.2};
+    context.setPositions(positions);
+    context.setVelocities(velocities);
+
+    // Run a short simulation and save a state..
+    integrator.step(500);
+    State savedState = context.getState(State::Positions | State::Velocities | State::IntegratorParameters); 
+
+    // Now continue the simulation
+    integrator.step(5);
+
+    // And try the same, starting from the state
+    newContext.setState(savedState);
+    newIntegrator.step(5);
+
+    State state1 = context.getState(State::Positions | State::Velocities);
+    State state2 = newContext.getState(State::Positions | State::Velocities);
+    ASSERT_EQUAL_VEC(state1.getPositions()[0], state2.getPositions()[0], 1e-6); 
+    ASSERT_EQUAL_VEC(state1.getPositions()[1], state2.getPositions()[1], 1e-6); 
+    ASSERT_EQUAL_VEC(state1.getVelocities()[0], state2.getVelocities()[0], 1e-6); 
+    ASSERT_EQUAL_VEC(state1.getVelocities()[1], state2.getVelocities()[1], 1e-6); 
+}
+
 void testAPIChangeNumParticles() {
     bool constrain = true;
     int numMolecules = 20;
@@ -553,6 +604,7 @@ int main(int argc, char* argv[]) {
         constrain = false; testDimerBox(constrain);
         constrain = true; testDimerBox(constrain);
         testCheckpoints();
+        testSaveParameters();
         testForceGroups();
         runPlatformTests();
     }

wrappers/python/simtk/openmm/app/simulation.py

@@ -312,7 +312,7 @@ class Simulation(object):
             a File-like object to write the state to, or alternatively a
             filename
         """
-        state = self.context.getState(getPositions=True, getVelocities=True, getParameters=True)
+        state = self.context.getState(getPositions=True, getVelocities=True, getParameters=True, getIntegratorParameters=True)
         xml = mm.XmlSerializer.serialize(state)
         if isinstance(file, str):
             with open(file, 'w') as f:

wrappers/python/src/swig_doxygen/swig_lib/python/extend.i

@@ -11,7 +11,8 @@
 
     def getState(self, getPositions=False, getVelocities=False,
                  getForces=False, getEnergy=False, getParameters=False,
-                 getParameterDerivatives=False, enforcePeriodicBox=False, groups=-1):
+                 getParameterDerivatives=False, getIntegratorParameters=False,
+                 enforcePeriodicBox=False, groups=-1):
         """Get a State object recording the current state information stored in this context.
 
         Parameters
@@ -28,6 +29,8 @@
             whether to store context parameters in the State
         getParameterDerivatives : bool=False
             whether to store parameter derivatives in the State
+        getIntegratorParameters : bool=False
+            whether to store integrator parameters in the State
         enforcePeriodicBox : bool=False
             if false, the position of each particle will be whatever position
             is stored in the Context, regardless of periodic boundary conditions.
@@ -64,6 +67,8 @@
             types += State.Parameters
         if getParameterDerivatives:
             types += State.ParameterDerivatives
+        if getIntegratorParameters:
+            types += State.IntegratorParameters
         state = _openmm.Context_getState(self, types, enforcePeriodicBox, groups_mask)
         return state