MULTIREFERENCE PERTURBATION-THEORY WITH FLEXIBLE ENERGY DENOMINATORS

Multireference perturbation theory (MRPT) is formulated with one-, two -, and higher-body shifts introduced into the energy denominators in a manner that preserves the linked diagram theorem (LDT) and the expone ntial form of the wave operator. Increased flexibility in the choice o f energy denominators permits perturbative expansions to have improved convergence behavior and avoid intruder states. This flexible-energy- denominator method can be combined with modified version of previous f ormulations to yield methods that have improved convergence behavior a nd/or retain the LDT. (C) 1998 American Institute of Physics.