Unitary group approach to spin-adapted open-shell coupled cluster theory
- Univ. of Warsaw (Poland)
- Univ. of Waterloo, Ontario (Canada)
- Pedagogical Univ., Czestochowa (Poland)
We show that the irreducible tensor operators of the unitary group provide a natural operator basis for the exponential Ansatz which preserves the spin symmetry of the reference state, requires a minimal number of independent cluster amplitudes for each substitution order, and guarantees the invariance of the correlation energy under unitary transformations of core, open-shell, and virtual orbitals. When acting on the closed-shell reference state with n{sub c} doubly occupied and n{sub v} unoccupied (virtual) orbitals, the irreducible tensor operators of the group U(n{sub c}) {circle_times} U(n{sub v}) generate all Gelfand-Tsetlin (GT) states corresponding to appropriate irreducible representation of U(n{sub c} + n{sub v}). The tensor operators generating the M-tuply excited states are easily constructed by symmetrizing products of M unitary group generators with the Wigner operators of the symmetric group S{sub M}. This provides an alternative to the Nagel-Moshinsky construction of the GT basis. Since the corresponding cluster amplitudes, which are also U(n{sub c}) {circle_times} U(n{sub v}) tensors, can be shown to be connected, the irreducible tensors operators of U(n{sub c}) {circle_times} U(n{sub v}) represent a convenient basis for a spin-adapted full coupled cluster calculation for closed-shell systems. For a high-spin reference determinant with n{sub s} singly occupied open-shell orbitals, the corresponding representation of U(n), n = n{sub c} + n{sub v} + n{sub s} is not simply reducible under the group U(n{sub c}) {circle_times} U(n{sub s}) {circle_times} U(n{sub v}). The multiplicity problem is resolved using the group chain U(n) {contains} U(n{sub c} + n{sub v}) {circle_times} U(n{sub s}) {contains} U(n{sub c}) {circle_times} U(n{sub v}). 78 refs., 7 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 478324
- Journal Information:
- International Journal of Quantum Chemistry, Vol. 56, Issue 3; Other Information: PBD: 5 Nov 1995
- Country of Publication:
- United States
- Language:
- English
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