Macroconfigurations in molecular electronic structure theory

Abstract

Based on the well-known idea of dividing a molecular orbital set into groups, and imposing restrictions on allowed group occupancies, a more general formalism, which we refer to as the macroconfiguration approach, is developed. The approach is shown to be useful in four complementary contexts: First, as with previous uses of orbital groupings, physical considerations can actively be taken into account in constructing configuration spaces; second, the approach is demonstrated to enable the screening of a vast number of noninteracting electron configuration pairs in a computationally efficient manner; third, all advantages of the Table-configuration interaction (CI) technique can be used effectively for evaluating Hamiltonian matrix elements between interacting configurations; finally, the macroconfiguration technique provides an efficient way of regenerating excited configurations as needed in a calculation. These characteristics indicate that the suggested approach is promising for improving the computational efficiency of methods describing electron dynamic correlation and, in particular, for CI calculations involving highly (e.g., triply and quadruply) excited configurations and for methods of effective Hamiltonian theory. The advantages of the suggested technique are illustrated by the results of calculations on the ground and excited states of the difluorodioxirane (CF 2O 2) molecule at the second-order generalized Van Vleck perturbation theory (GWPT2) level. © 2004 Wiley Periodicals, Inc.