Fix bug where CHARMM impropers did not respect toroidal boundary conditions

devtools/packaging/README.md

 # Packaging OpenMM into ZIP installers
 
+Set your environment variable `TAG` to the git tag for the release:
+```bash
+# OpenMM 7.1.0rc1
+export TAG="9567ddb"
+```
+
 ## Source
 
 Start the docker container:
 ```bash
-docker run -i -t --rm -v `pwd`:/io jchodera/omnia-build-box:cuda80-amd30-clang38 bash
+docker run -i -t --rm -e TAG -v `pwd`:/io jchodera/omnia-build-box:cuda80-amd30-clang38 bash
 ```
 Inside the docker container:
 ```bash
@@ -23,7 +29,7 @@ cp packaging/compressed/* /io
 
 Start the docker container:
 ```bash
-docker run -i -t --rm -v `pwd`:/io jchodera/omnia-build-box:cuda80-amd30-clang38 bash
+docker run -i -t --rm -e TAG -v `pwd`:/io jchodera/omnia-build-box:cuda80-amd30-clang38 bash
 ```
 Inside the docker container:
 ```bash
@@ -50,4 +56,3 @@ source openmm/devtools/packaging/scripts/osx/prepare.sh
 source openmm/devtools/packaging/scripts/osx/build.sh
 source openmm/devtools/packaging/scripts/osx/package.sh
 ```
-

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

@@ -906,8 +906,11 @@ class CharmmPsfFile(object):
 
         if verbose: print('Adding impropers...')
         # Ick. OpenMM does not have an improper torsion class. Need to
-        # construct one from CustomTorsionForce
-        force = mm.CustomTorsionForce('k*(theta-theta0)^2')
+        # construct one from CustomTorsionForce that respects toroidal boundaries
+        energy_function  = 'k*dtheta_torus^2;'
+        energy_function += 'dtheta_torus = dtheta - floor(dtheta/(2*pi)+0.5)*(2*pi);'
+        energy_function += 'dtheta = theta - theta0;'
+        force = mm.CustomTorsionForce(energy_function)
         force.addPerTorsionParameter('k')
         force.addPerTorsionParameter('theta0')
         force.setForceGroup(self.IMPROPER_FORCE_GROUP)