ANALYTIC EVALUATION OF ENERGY GRADIENTS AT THE COUPLED-CLUSTER SINGLES AND DOUBLES LEVEL USING QUASI-RESTRICTED HARTREE-FOCK OPEN-SHELL REFERENCE FUNCTIONS

Analytic formulas for the evaluation of energy gradients at the coupled-cluster singles and doubles (CCSD) level for a special class of open-shell quasi-restricted Hartree-Fock (QRHF) reference functions are presented and their computational implementation is discussed. The reference functions considered here are those which are formed from a closed-shell determinant for the molecule in a different ionization state by either adding or removing a single electron. Due to the approximate invariance of the CCSD model with respect to the choice of orbitals, results obtained with QRHF-CCSD closely approximate those from CCSD calculations using unrestricted Hartree-Fock (UHF) or restricted open-shell Hartree-Fock (ROHF) reference functions in cases where the latter references are well behaved. However, in cases where spin contamination (UHF) or symmetry breaking (UHF and ROHF) effects are important, QRHF-CCSD offers an attractive alternative since the resulting wave function is a projected eigenfunction of the spin operator S2 and transforms as a pure irreducible representation of the molecular point group.