Accurate Noncovalent Interaction Energies Using Truncated Basis Sets Based on Frozen Natural Orbitals.

We assess the accuracy of basis set truncations based on natural orbitals determined by second-order perturbation theory for computing noncovalent interaction energies with coupled cluster through perturbative triples [CCSD(T)]. We consider two methods for truncation: (i) the usual frozen natural orbital approach (FNO) in which the basis set truncation occurs before the iterative CCSD computation [FNO CCSD(T)] and (ii) an approach in which the truncation occurs only for the perturbative triples contribution [CCSD+FNO(T)]. The errors incurred are comparable for both methods and are small enough for the methods to be used for benchmark-quality studies of noncovalent interactions. For the FNO CCSD(T) method with a modest natural orbital occupation tolerance of 10(-5), the mean absolute error in the interaction energies for the S22 data set in an aug-cc-pVDZ basis set is only 0.012 kcal mol(-1) versus canonical CCSD(T) values. The same method exhibits a mean absolute error of 0.020 kcal mol(-1) for the S11 data set in the aug-cc-pVTZ basis set versus canonical CCSD(T) values.