A WAVE-PACKET APPROACH TO THE LIOUVILLE-VON NEUMANN EQUATION FOR DISSIPATIVE SYSTEMS

A method for the numerical solution of the dissipative Liouville-von N eumann equation is presented. The reduced density operator is expanded in a basis of time-dependent wave functions. This guarantees that the size of the basis required in the representation of the density opera tor is minimal. Equations of motion for the expansion wave functions a nd the density matrix elements are obtained from the Dirac-Frenkel var iational principle. The numerical effort of the method scales proporti onal to N, where N is the dimension of the relevant Hilbert space. As a first example, the dynamics of a three-mode system with vibronic cou pling (with N approximate to 50 000) coupled to a dissipative bath has been studied, modeling the S-1/S-2 states of pyrazine. For the cw-abs orption spectrum, fast convergence with respect to the number of expan sion wave functions has been obtained. (C) 1997 American Institute of Physics.