A few more changes related to setIntegrationForceGroups()

openmmapi/include/openmm/LocalEnergyMinimizer.h

@@ -43,6 +43,10 @@ namespace OpenMM {
  * force to the potential function.  The strength of the restraining force is steadily increased
  * until the minimum energy configuration satisfies all constraints to within the tolerance
  * specified by the Context's Integrator.
+ * 
+ * Energy minimization is done using the force groups defined by the Integrator.
+ * If you have called setIntegrationForceGroups() on it to restrict the set of forces
+ * used for integration, only the energy of the included forces will be minimized.
  */
 
 class OPENMM_EXPORT LocalEnergyMinimizer {

tests/TestBrownianIntegrator.h

@@ -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) 2008-2015 Stanford University and the Authors.      *
+ * Portions copyright (c) 2008-2020 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -31,6 +31,7 @@
 
 #include "openmm/internal/AssertionUtilities.h"
 #include "openmm/Context.h"
+#include "openmm/CustomExternalForce.h"
 #include "openmm/HarmonicBondForce.h"
 #include "openmm/NonbondedForce.h"
 #include "openmm/System.h"
@@ -279,6 +280,31 @@ void testInitialTemperature() {
     ASSERT_USUALLY_EQUAL_TOL(targetTemperature, temperature, 0.01);
 }
 
+void testForceGroups() {
+    System system;
+    system.addParticle(1.0);
+    BrownianIntegrator integrator(0, 1.0, 0.001);
+    integrator.setIntegrationForceGroups(1<<1);
+    CustomExternalForce* f1 = new CustomExternalForce("x");
+    f1->addParticle(0);
+    f1->setForceGroup(1);
+    CustomExternalForce* f2 = new CustomExternalForce("y");
+    f2->addParticle(0);
+    f2->setForceGroup(2);
+    system.addForce(f1);
+    system.addForce(f2);
+    Context context(system, integrator, platform);
+    context.setPositions(vector<Vec3>(1));
+
+    // Take one step and verify that the position was updated based only on f1.
+
+    integrator.step(1);
+    Vec3 pos = context.getState(State::Positions).getPositions()[0];
+    ASSERT(pos[0] < 0);
+    ASSERT(pos[1] == 0);
+    ASSERT(pos[2] == 0);
+}
+
 void runPlatformTests();
 
 int main(int argc, char* argv[]) {
@@ -290,6 +316,7 @@ int main(int argc, char* argv[]) {
         testConstrainedMasslessParticles();
         testRandomSeed();
         testInitialTemperature();
+        testForceGroups();
         runPlatformTests();
     }
     catch(const exception& e) {

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

@@ -106,6 +106,33 @@ Parameters:
   }
 }
 
+%extend OpenMM::Integrator {
+  %pythoncode %{
+    def setIntegrationForceGroups(self, groups):
+        """Set which force groups to use for integration.  By default, all force groups are included.
+
+        Parameters
+        ----------
+        groups : set or int
+            a set of indices for which force groups to include when integrating the equations of motion.
+            Alternatively, the groups can be passed as a single unsigned integer interpreted as a bitmask,
+            in which case group i will be included if (groups&(1<<i)) != 0.
+        """
+        try:
+            # is the input integer-like?
+            groups_mask = int(groups)
+        except TypeError:
+            if isinstance(groups, set):
+                groups_mask = functools.reduce(operator.or_,
+                        ((1<<x) & 0xffffffff for x in groups))
+            else:
+                raise TypeError('%s is neither an int nor set' % groups)
+        if groups_mask >= 0x80000000:
+            groups_mask -= 0x100000000
+        _openmm.Integrator_setIntegrationForceGroups(self, groups_mask)
+    %}
+}
+
 %extend OpenMM::RPMDIntegrator {
   %pythoncode %{
     def getState(self,