Also cache proper torsion search.

Has a much smaller effect compared to caching the improper search, but non-negligible and easy to do.

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

@@ -1968,12 +1968,15 @@ class PeriodicTorsionGenerator(object):
         else:
             force = existing[0]
         wildcard = self.ff._atomClasses['']
+        proper_cache = {}
         for torsion in data.propers:
-            type1 = data.atomType[data.atoms[torsion[0]]]
-            type2 = data.atomType[data.atoms[torsion[1]]]
-            type3 = data.atomType[data.atoms[torsion[2]]]
-            type4 = data.atomType[data.atoms[torsion[3]]]
-            match = None
+            type1, type2, type3, type4 = [data.atomType[data.atoms[torsion[i]]] for i in range(4)]
+            sig = (type1, type2, type3, type4)
+            sig = frozenset((sig, sig[:-1]))
+            match = proper_cache.get(sig, None)
+            if match == -1:
+                continue
+            if match is None:
                 for index in self.propersForAtomType[type2]:
                     tordef = self.proper[index]
                     types1 = tordef.types1
@@ -1986,6 +1989,10 @@ class PeriodicTorsionGenerator(object):
                             match = tordef
                         if not hasWildcard:
                             break
+                if match is None:
+                    proper_cache[sig] = -1
+                else:
+                    proper_cache[sig] = match
             if match is not None:
                 for i in range(len(match.phase)):
                     if match.k[i] != 0: