There seem to be some discrepancies in Open Babel's implementation of UFF for molecules containing phenyl groups. Namely, when obminimize is used to find the minimum energy structure of biphenyl, it fails to reproduce the dihedral angle between the two rings as reported by Casewit et. al in "Application of a Universal Force Field to Organic Molecules." For comparison, Open Babel finds the dihedral angle between the rings to be 3.6 *°,* while Casewit et al report that UFF found it to be 41.7*°* which is much closer to that determined in experiments. Other phenyl containing molecules, like stilbene, seem to have the same problem. Perhaps Open Babel is not assigning bond orders correctly?
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