Optimized auxiliary basis sets for explicitly correlated methods.

Auxiliary basis sets for use in explicitly correlated MP2-F12 and CCSD-F12 methods, in which three- and four-electron integrals are approximated as products of two-electron integrals through the resolution of the identity (RI), have been optimized for the elements H, B-Ne, and Al-Ar. Fully matched to the recently constructed cc-pVnZ-F12 orbital basis sets, these new auxiliary basis sets result in very small RI errors, as exemplified by the calculated atomization energies of 42 molecules at the MP2-F12 level. Their utility in calculating smooth potential energy surfaces is also demonstrated in calculations of the spectroscopic properties of several diatomic molecules.