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Achieving the CCSD(T) BasisSet Limit in Sizable Molecular Clusters: Counterpoise Corrections for the ManyBody Expansion
"... ABSTRACT: An efficient procedure is introduced to obtain the basisset limit in electronic structure calculations of large molecular and ionic clusters. This approach is based on a Boys−Bernardistyle counterpoise correction for clusters containing arbitrarily many monomer units, which is rendered c ..."
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ABSTRACT: An efficient procedure is introduced to obtain the basisset limit in electronic structure calculations of large molecular and ionic clusters. This approach is based on a Boys−Bernardistyle counterpoise correction for clusters containing arbitrarily many monomer units, which is rendered computationally feasible by means of a truncated manybody expansion. This affords a tractable way to apply the sequence of correlationconsistent basis sets (augccpVXZ) to large systems and thereby obtain energies extrapolated to the complete basis set (CBS) limit. A threebody expansion with threebody counterpoise corrections is shown to afford errors of ≲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, δCCSD(T) = ECCSD(T) − EMP2, 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. SECTION: Molecular Structure, Quantum Chemistry, and General Theory For electronic structure calculations of dimer bindingenergies (BEs), it is wellknown that naıv́e attempts to use the formula = − −E E EBE AB A B (1) usually result in significant overestimation of the A···B binding energy, owing to the basisset superposition error (BSSE) that disappears only very slowly as the monomer basis sets approach completeness. BSSE is a manifestation of the fact that eq 1 is an unbalanced approximation in the sense that EAB is computed using a more flexible basis set than is used to compute the monomer energies. The widely used Boys−Bernardi counterpoise (CP) procedure1 corrects this imbalance by computing all three energies using the dimer (AB) basis set. The obvious generalization of this CP correction to Nbody clusters is2−4