Approaches to the approximate treatment of complex molecular systems by the multiconfiguration time-dependent Hartree method

A consistent treatment of environmental effects is proposed in the framewor k of the multiconfiguration time-dependent Hartree (MCTDH) method. The meth od is extended in view of treating complex molecular systems which require an exact quantum dynamics for a certain number of "primary" modes while an approximate dynamics is adequate for a class of "secondary" modes. The latt er may correspond to the weakly coupled modes in a polyatomic molecule, or the first solvent shell in a solute-solvent complex. For these modes, a des cription in terms of parameterized functions is introduced. The MCTDH worki ng equations are generalized to allow for the nonorthogonality of these fun ctions, which may take, e.g., a multidimensional Gaussian form. The formali sm is developed on the level of both the wave function description and the density matrix description. Dissipative effects are accounted for in terms of a stochastic Hamiltonian approach versus master equation approach in the respective descriptions. (C) 1999 American Institute of Physics. [S0021-96 06(99)51730-3].