Achieving the CCSD(T) Basis-Set Limit in Sizable Molecular Clusters: Counterpoise Corrections for the Many-Body Expansion

An efficient procedure is introduced to obtain the basis-set limit in electronic structure calculations of large molecular and ionic clusters. This approach is based on a Boys-Bernardi-style counterpoise correction for clusters containing arbitrarily many monomer units, which is rendered computationally feasible by means of a truncated many-body expansion. This affords a tractable way to apply the sequence of correlation-consistent basis sets (aug-cc-pVXZ) to large systems and thereby obtain energies extrapolated to the complete basis set (CBS) limit. A three-body expansion with three-body counterpoise corrections is shown to afford errors of less than or similar to 0.1-0.2 kcal/mol with respect to traditional MP2/CBS results, even for challenging systems such as fluoride-water clusters. A triples correction, delta(CCSD(T)) = E-CCSD(T) - E-MP2, can be estimated accurately and efficiently as well. Because the procedure is embarrassingly parallelizable and requires no electronic structure calculations in systems larger than trimers, it is extendible to very large clusters. As compared to traditional CBS extrapolations, computational time is dramatically reduced even without parallelization.