Numerical solution of the coupled-cluster single- and double-excitation equations with application to Be andLi−

An implementation of the coupled-cluster single- and double-excitation (CCSD) method is presented. The CCSD equations are expressed in form of a system of coupled one-particle and two-particle equations. The complete expressions of these equations in terms of Goldstone diagrams are given. Also presented is a simplified diagrammatic notation corresponding to an efficient evaluation scheme. By using numerical discrete spectra to solve these equations, a high numerical accuracy, better than 1 μhartree, is obtained. The method is tested on Be and ${\mathrm{Li}}^{\mathrm{-}}$ and is found to account for 99.3% and 99.4% of the correlation energy, respectively. The CCSD method is demonstrated to give total energies depending very little on the potential used to define the perturbation expansion.