ACCSD(T) Energies and Forces: Improving Upon CCSD(T)

Coupled-cluster theory with singles, doubles and perturbative triples [CCSD(T)] is a high-accuracy reference to which other methods are measured; however, away from equilibrium RHF based CCSD(T) often fails. To remedy this failure, the LambdaCCSD(T) method was introduced and implemented for both energies and gradients within the ACESII program system. Like CCSD(T), LambdaCCSD(T) is extensive, orbitally invariant, and scales O(N7). Applying LambdaCCSD(T) to molecules at equilibrium shows little degradation of the CCSD(T) results. For bond-breaking LambdaCCSD(T) is shown to be a significant improvement over CCSD(T).