Multiconfigurational expansions of density operators: equations of motion and their properties

Multiconfigurational expansions of density operators which may be used in n umerical treatments of the dynamics of closed and open quantum systems are introduced. The expansions of the density operators may be viewed as analog ues of those used in the multiconfiguration time-dependent Martreee (MCTDH) method, which is a well-established and highly efficient method for propag ating wavepackets in several dimensions. There is no unique multiconfigurat ional representation of a density operator and two sensible types of MCTDH- like expansions are studied. Equations of motion for these multiconfigurati onal expansions are presented by adopting the Dirac-Frenkel/McLachlan varia tional principle (or variants of thereof). Various properties of these sets of equations of motion are derived for closed and open system dynamics. Th e numerical and technical aspects of this approach have been recently discu ssed by us [(1999) J Chem Phys 111. 8759]. Here we discuss the formal aspec ts of the approach in a more general context.