for models I-IV, using both fixed-atom and harmonic restraints. All values include ZPE corrections, which are also listed separately in Table . In the case of fixedatom constraints, results are compared for two different Hessians, one that is unmodified and one in which the fixed-atom contributions are deleted, as in eq. 3. Small imaginary frequencies are observed with the unmodified Hessian, which are absent when eq. 3 is used instead, although neither ∆ ‡ E nor ∆ rxn E is significantly affected by the presence or absence of these imaginary frequencies. The use of harmonic restraints affords structures that are strictly free of imaginary frequencies save for the overthe-barrier mode at the transition state.