On the one-particle basis set relaxation in R12 based theories

When the explicitly correlated R12/F12 theories are applied in combination with smaller one-particle orbital basis sets, the relaxation with respect to the one-particle limit basis becomes relevant. Starting from a single reference Slater determinant in a given computational orbital basis, two ways of coping with this effect within the coupled cluster R12 theory are compared. First is based on the full treatment of the correlation factor including its formal one-particle component, the second is based on a separate exponential expansion in terms of single excitations using a dual Hartree-Fock limit basis. Although, from the computational point of view the first approach would be preferable, its accuracy performance is somewhat worse. Using fixed one-particle basis set relaxed amplitudes in subsequent coupled cluster solution may be recommended. (C) 2008 Elsevier B.V. All rights reserved.