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For the fused heterocycle, puckering in QM (green) is not reproduced well in the MM (cyan) optimized geometry, which remains flat. One of the bridgehead carbons (atom 10 in 2D depiction) can be seen to be out of plane in QM and almost in the plane of the ring with nitrogens for MM. We observe similar behavior in several other cases of bridgehead atoms with heteroatom neighbors.
