Basis set converged weak interaction energies from conventional and explicitly correlated coupled-cluster approach
Interaction energies for seven weakly bound dimers involving helium, argon, water, and methane are computed using large correlation-consistent basis sets augmented with bond functions. The estimates of the coupled-cluster singles, doubles, and noniterative triples [CCSD(T)] complete basis set limit are obtained using both the conventional approach and several variants of the explicitly correlated CCSD(T)-F12 method. It is shown that both bond functions and the F12 approach significantly speed up the convergence of the CCSD(T)/aug-cc-pVXZ interaction energies with the basis set cardinal number X. However, the extent of improvement provided by each technique varies with the character of the interactions—the F12 method works best for polar, electrostatics-bound dimers, while for dispersion-dominated complexes the addition of bond functions is more efficient. The convergence rate afforded by different coupled-cluster variants is fairly consistent across the entire attractive region of the potential curve, while the improvement provided by the F12 correction increases along the repulsive wall. The use of large basis sets and the agreement between conventional and explicitly correlated approaches allow us to assess the importance of different residual approximations present in the popular CCSD(T)-F12 implementations.