Cd. Sherrill et al., ENERGIES AND ANALYTIC GRADIENTS FOR A COUPLED-CLUSTER DOUBLES MODEL USING VARIATIONAL BRUECKNER ORBITALS - APPLICATION TO SYMMETRY-BREAKINGIN O-4(+), The Journal of chemical physics, 109(11), 1998, pp. 4171-4181
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.