Large, correlation-consistent basis sets have been used to very closely approximate the coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] complete basis set potential energy curves of several prototype nonbonded complexes, the sandwich, T-shaped, and parallel-displaced benzene dimers, the methane-benzene complex, the H2S-benzene complex, and the methane dimer. These benchmark potential energy curves are used to assess the performance of several methods for nonbonded interactions, including various spin-component-scaled second-order perturbation theory (SCS-MP2) methods, the spin-component-scaled coupled-cluster singles and doubles method (SCS-CCSD), density functional theory empirically corrected for dispersion (DFT-D), and the meta-generalized-gradient approximation functionals M05-2X and M06-2X. These approaches generally provide good results for the test set, with the SCS methods being somewhat more robust. M05-2X underbinds for the test cases considered, while the performances of DFT-D and M06-2X are similar. Density fitting, dual basis, and local correlation approximations all introduce only small errors in the interaction energies but can speed up the computations significantly, particulary when used in combination.

The S22 test set of interaction energies for small model complexes [Phys. Chem. Chem. Phys. 8, 1985 (2006)] has been very valuable for benchmarking new and existing methods for noncovalent interactions. However, the basis sets utilized to compute the CCSD(T) interaction energies for some of the dimers are insufficient to obtain converged results. Here we consistently extrapolate all CCSD(T)/complete basis set (CBS) interaction energies using larger basis sets for the CCSD(T) component of the computation. The revised values, which we designate S22A, represent the most accurate results to date for this set of dimers. The new values appear to be within a few hundredths of 1 kcal mol(-1) of the true CCSD(T)/CBS limit at the given geometries, but the former S22 values are off by as much as 0.6 kcal mol(-1) compared to the revised values. Because some of the most promising methods for noncovalent interactions are already achieving this level of agreement (or better) compared to the S22 data, more accurate benchmark values would clearly be helpful. The MP2, SCS-MP2, SCS-CCSD, SCS(MI)-MP2, and B2PLYP-D methods have been tested against the more accurate benchmark set. The B2PLYP-D method outperforms all other methods tested here, with a mean average deviation of only 0.12 kcal mol(-1). However, the consistent, slight underestimation of the interaction energies computed by the SCS-CCSD method (an overall mean absolute deviation and mean deviation of 0.24 and -0.23 kcal mol(-1), respectively) suggests that the SCS-CCSD method has the potential to become even more accurate with a reoptimization of its parameters for noncovalent interactions.

There has been much interest in cost-free improvements to second-order Møller-Plesset perturbation theory (MP2) via scaling the same- and opposite-spin components of the correlation energy (spin-component scaled MP2). By scaling the same- and opposite-spin components of the double excitation correlation energy from the coupled-cluster of single and double excitations (CCSD) method, similar improvements can be achieved. Optimized for a set of 48 reaction energies, scaling factors were determined to be 1.13 and 1.27 for the same- and opposite-spin components, respectively. Preliminary results suggest that the spin-component scaled CCSD (SCS-CCSD) method will outperform all MP2 type methods considered for describing intermolecular interactions. Potential energy curves computed with the SCS-CCSD method for the sandwich benzene dimer and methane dimer reproduce the benchmark CCSD(T) potential curves with errors of only a few hundredths of 1 kcal mol(-1) for the minima. The performance of the SCS-CCSD method suggests that it is a reliable, lower cost alternative to the CCSD(T) method.

An examination of the performance of density-fitted, spin-component-scaled, second-order Møller-Plesset theory (SCS-MP2), SCS-MP2 with parameters optimized for nucleic acids (SCSN-MP2), and their local-correlation variants, SCS-LMP2 and SCSN-LMP2, is presented for the sandwich and T-shaped benzene dimers, the methane-benzene and H(2)S-benzene complexes, and the methane dimer over entire potential energy curves. These are compared to benchmark-quality estimates of the complete-basis-set limit for coupled-cluster theory through perturbative triple excitations, CCSD(T)/CBS. With the exception of the methane dimer, SCSN-LMP2/CBS tends to outperform SCS-LMP2/CBS with maximum relative errors of 6 and 18%, respectively, at the optimal CCSD(T)/CBS intermolecular distances. For the methane dimer, errors for SCS(N)-(L)MP2/CBS remain in the 0.2-0.3 kcal mol(-1) range, corresponding to a larger relative error of 40-50%. Although the local MP2 methods perform very similarly to their conventional counterparts when aug-cc-pVTZ or larger basis sets are used, in the absence of counterpoise correction the local approximation becomes significantly worse for the aug-cc-pVDZ basis set. The changes due to local correlation approximations for the aug-cc-pVDZ basis are reduced when diffuse functions are neglected for hydrogen atoms.

Noncovalent interactions play a significant role in determining the structures of DNA, RNA, and proteins. Among the most prevalent are pi-pi interactions, which occur as favorable forces between the aromatic subunits of biochemical molecules. The aromatic side chains of amino acids tryptophan and phenylalanine are commonly modeled with indole and benzene, respectively. We have utilized the MP2 and SCS-MP2 methods with the aug-cc-pVDZ basis set to compute several T-shaped interaction energies and parallel displaced (PD) three-dimensional potential energy surfaces (PESs) at 3.4, 3.6, and 3.8 A vertical separations. At selected minima, CCSD(T) results extrapolated to the complete-basis-set (CBS) limit were obtained. The trend of the T-shaped interactions has been rationalized by considering electrostatic potential maps and symmetry-adapted perturbation theory (SAPT) results. The global minimum has been verified to be the N-H/pi T-shaped configuration with a CCSD(T)/CBS interaction energy of -5.62 kcal mol(-1). For the PD PESs, the MP2 and SCS-MP2 methods predict different minimum configurations. The CCSD(T) method favors the SCS-MP2 PD configuration over the MP2 PD configuration by 0.18 kcal mol(-1). Among the approximate methods considered here, the SCS-CCSD method extrapolated to the CBS limit incurs only around 2% error compared to CCSD(T)/CBS results and is the most reliable for the interaction energies of the indole-benzene complex. Overall, the extension of aromaticity and the highly positive hydrogen of the N-H bond, both exhibited by indole, enhance the strength of nonbonded interactions with benzene compared to those in the benzene dimer or in the pyridine-benzene complex.