Table summarizes the results of relaxed-constrained vibrational frequency calculations, including the total ZPE and also the number of imaginary frequencies obtained, n im . In two cases, a single imaginary frequency remains when harmonic confining potentials are used, typically associated with the methyl group on residue B and which is an artifact of the dihedral constraints that are used in these dipeptide examples; these constraints will be lifted for the realistic enzyme models that are considered below. With fixed-atom constraints and an unmodified Hessian, 10 of 12 examples exhibit n im = 1 or 2, suggesting that most of these originate from the use of anchor atoms rather than the dihedral constraints. When the zeroed-out Hessian is used at the same fixed-atom relaxed geometry, however, the number of imaginary frequencies proliferates. Defining