Application of adiabatic switching to vibrational energies of three-dimensional HCO, H2O, and H2CO

Abstract

Semiclassical vibrational energies are calculated for nonrotating, three-dimensional HCO, H2O, and H2CO by the adiabatic switching method. For HCO and H2O the Hamiltonian is given in a body-fixed frame in terms of Jacobi coordinates. Several zero-order Hamiltonians for adiabatic switching to the full Hamiltonian are considered. The simple, ‘‘obvious’’ choices do not yield stable results; however, a nonseparable zero-order Hamiltonian, which can be transformed to a simple normal form, does give stable results. Semiclassical quantization is done with the transformed zero-order Hamiltonian, initial conditions are obtained in terms of the Jacobi coordinates, and adiabatic switching is done in these coordinates. The Watson Hamiltonian is used for H2CO in a straightforward fashion. In all cases, the semiclassical energies are in good agreement with quantum mechanical results, and for H2O in excellent agreement with other semiclassical results.