Comparison of self-consistent field treatments of kinetic couplings in calculations on polyatomic molecules

The accuracy of the vibrational self-consistent field (VSCF) and time-dependent self-consistent field (TDSCF) approximations for studies of vibrational dynamics of polyatomic systems is investigated for systems in which the most important off-diagonal coupling terms in the Hamiltonian are in the kinetic energy operator. To lowest order, these terms are zero in the VSCF treatment. This leads to large differences between the energies calculated using this approximation, compared to those obtained from a variational calculation. In contrast, the energy levels, obtained using the TDSCF approximation, are in good agreement with variational results. These questions are investigated using a collinear model of HCN in an internal coordinate representation.