The performance of the explicitly correlated coupled cluster method. I. The four‐electron systems Be, Li⁻, and LiH Available to Purchase

To study the reliability of the recently developed explicitly correlated coupled cluster method (CCSDT1‐R12) we have performed calculations on the four‐electron systems Be, Li⁻, and LiH on various levels of perturbation theory and coupled cluster theory with and without explicit linear r_ij terms. The convergence of the total energy with increasing size of the basis is much faster than for the conventional coupled cluster approach. Our CCSDT1‐R12 energies of −14.667261 E_h for Be and −7.500671 E_h for the Li⁻ ground state are the best ones computed so far and are close to previous estimates of the CCSDT 1 basis set limits. The Be result differs from the ‘‘experimental’’ nonrelativistic energy by ca. 0.1 mE_h, mainly due to neglect of quadruple excitations. Our Born–Oppenheimer energy of LiH at the equilibrium distance of −8.070487 E_h is close to the experimental nonrelativistic energy. The binding energy (D₀) of LiH with respect to Li⁺ and H⁻ is calculated as −7.152 eV, in agreement with the experimental value within a meV. For LiH the harmonic vibrational frequencies and other related spectroscopic constants are studied in their basis dependence as well. The equilibrium distance and the harmonic vibrational frequency of LiH are much less sensitive to the inclusion of terms that explicitly depend on the interelectronic coordinates. Basis set superposition errors are much smaller in the R12 approach than in the conventional calculations, especially for the smaller basis sets.