A unitary group formulation of the complete active space configuration interaction method. I. General formalism

Gould, M. D.; Chandler, G. S.
April 1989
Journal of Chemical Physics;4/1/1989, Vol. 90 Issue 7, p3680
Academic Journal
A group theoretical formulation of the complete active space CI (CASCI) method is presented based on the subgroup chain U(n=n1+n2)[contains]U(n1)×U(n2), where n1 is the number of active orbitals and n2 the number of external orbitals. The internal-external factorization of the U(n) generator matrices and the isolation of all singly, doubly (and higher) excited states fall out naturally in our approach. Moreover it is shown that the matrix elements of arbitrary products of U(n) generators may be obtained from simple products of the single generator matrices, to any order of excitation. The method is ideally suited to modern vector array processors and affords sufficient flexibility to allow core excitations and partitioning of the active space. Further simplifications occur in the all singles and doubles case, which will be discussed in the second (and final) paper of the series.


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