ENERGIES AND ANALYTIC GRADIENTS FOR A COUPLED-CLUSTER DOUBLES MODEL USING VARIATIONAL BRUECKNER ORBITALS - APPLICATION TO SYMMETRY-BREAKINGIN O-4(+)

We describe an alternative procedure for obtaining approximate Brueckn er orbitals in ab initio electronic structure theory. Whereas approxim ate Brueckner orbitals have traditionally been obtained by mixing the orbitals until the coefficients of singly substituted determinants in the many-electron wave function become zero, we remove singly substitu ted determinants at the outset and obtain orbitals which minimize the total electronic energy. Such orbitals may be described as variational Brueckner orbitals. These two procedures yield the same set of exact Brueckner orbitals in the full configuration interaction limit but dif fer for truncated wave functions. We consider the simplest variant of this approach in the context of coupled-cluster theory, optimizing orb itals for the coupled-cluster doubles (CCD) model. An efficient new me thod is presented for solving the coupled equations defining the energ y, doubles amplitudes, and orbital mixing parameters. Results for seve ral small molecules indicate nearly identical performance between the traditional Brueckner CCD method and the variational Brueckner orbital CCD approach. However, variational Brueckner orbitals offer certain a dvantages: they simplify analytic gradients by removing the need to so lve the coupled-perturbed Brueckner coupled-cluster equations for the orbital response, and their straightforward extensions for inactive or bitals suggests possible uses in size-extensive models of nondynamical electron correlation. Application to O-4(+) demonstrates the utility of variational Brueckner orbitals in symmetry breaking cases. (C) 1998 American Institute of Physics.