A unitary group formulation of the complete active space configuration interaction method. II. An approach based on the subgroup chain U(n=n+n1+n2) = U(n)×U(n1 +in2) = U(n)×U(n1)×U(n2)

Abstract

In this paper, we examine some aspects of complete active space configuration interaction (CASCI) calculations where excitations from the core are taken into account. We show that it is appropriate to work in a basis for the irreducible representations (irreps) of the orbital unitary group symmetry adapted to the subgroup U(n0)×U(n1)×U(n2) (where n0, n1, and n2 are the number of core, active, and external orbitals, respectively, and n=n0+n1+n2). We demonstrate that this approach has the following advantages; (i) it allows the efficient isolation of single, double, and higher excitations from the reference space; (ii) it demonstrates explicitly the internal–external factorization of generator matrix elements; (iii) it allows the determination of the matrix elements of arbitrary generator products by simple matrix multiplications.