Dispersion Energy of Symmetry-Adapted Perturbation Theory from the Explicitly Correlated F12 Approach.

Methods of the explicitly correlated F12 approach are applied to the problem of calculating the uncoupled second-order dispersion energy in symmetry-adapted perturbation theory. The accuracy of the new method is tested for noncovalently bound complexes from the A24 data set [J. Řezáč and P. Hobza, J. Chem. Theory Comput. 2013, 9, 2151] using standard orbital basis sets aug-cc-pV XZ supplemented with auxiliary aug-cc-pV XZ_OPTRI sets. For near equilibrium geometries, it is possible to recover the dispersion energy with average relative errors consistently smaller than 0.1% (with respect to the CBS extrapolated limit estimated from regular orbital calculations). This level of accuracy is achieved already in the basis set of a triple-ζ quality, when a Slater-type correlation factor exp(-0.9 r12) is combined with variant C of the F12 approach. The explicitly correlated approach clearly outperforms regular orbital calculations in the basis set of quintuple-ζ quality (average relative errors of 1%).