Adiabatic Switching Extended To Prepare Semiclassically Quantized Rotational-Vibrational Initial States for Quasiclassical Trajectory Calculations.

An approximation-free adiabatic switching method to generate semiclassically quantized ensembles of rovibrational states of polyatomic molecules for use as initial conditions in quasiclassical trajectory calculations is presented. Vibrational states are prepared, starting from an ensemble of classical states corresponding to the desired quantum state of the normal-mode Hamiltonian by slowly switching on the anharmonicity in internal coordinates, thereby avoiding rotational contamination. To generate rovibrational states, an extension is proposed: The vibrationally quantized molecules are slowly spun up to the desired quantized angular momentum. The ensembles obtained with adiabatic switching for CH4 are insensitive to the choice of internal coordinates and stationary; furthermore, their mean energies agree remarkably well with the quantum mechanical values: The zero-point energy and 15 vibrational levels of the first three polyads are within 20 cm-1, the rotational levels are between J = 1 and 50 within 1%, and the standard deviation is always <1%. Adiabatic switching produces classical state ensembles with significantly better properties than normal-mode sampling, making them more appropriate in quasiclassical trajectory calculations.