Abstract
The Brueckner doubles variant of coupled cluster theory has recently been reintroduced by the authors. The use of Brueckner orbitals means that the governing equations for T̂2 take a particularly simple form. Here we give the details for the evaluation of the gradient of the Brueckner doubles energy for (a) the unrestricted spin–orbital formalism and (b) the closed-shell restricted formalism. Applications are presented for H2O, NH3, CH4, H2CO, C2H2, HCN, and CO2 and comparisons are made with the Hartree–Fock, second order Mo/ller–Plesset and quadratic configuration interaction models and with experiment.
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