Quantum dynamics of large polyatomic systems using a classically based separable potential method

A new method for approximate solution of the time-dependent vibrational Schrödinger equation, applicable to extended molecular systems, is presented. The new method is essentially an approximate time-dependent quantization of classical dynamics. A molecular dynamics simulation is used to obtain a separable, effective time-dependent potential for each mode, that implicitly includes also the effects of all the other modes on this degree of freedom. A time-dependent wave packet is then propagated separately for each mode, using the corresponding effective potential. The new approximation is valid for short time scale processes only, but it is easily applicable to large realistic systems. Test calculations against exact quantum and time-dependent self-consistent field (TDSCF) results are carried out for two examples; photodissociation of HI in the collinear Xe⋯HI cluster, and electron photodetachment from the collinear Ar⋯I^-⋯Ar cluster. For illustration, the new scheme is also applied to photodetachment from large linear clusters Ar_n⋯I^-⋯Ar_n (n = 2-8) and the results are discussed. For the test systems, the results of the new method are virtually identical to those following from the computationally much more demanding TDSCF approach, and they are in excellent agreement with the exact results.