Abstract
The performance of the [2]S and [2]R12 universal perturbative corrections that account for one- and many-body basis set errors of single- and multiconfiguration electronic structure methods is assessed. A new formulation of the [2]R12 methods is used in which only strongly occupied orbitals are correlated, making the approach more amenable for larger computations. Three model problems are considered using the aug-cc-pVXZ (X = D,T,Q) basis sets: the electron affinity of fluorine atom, a conformational analysis of two Si2H4 structures, and a description of the potential energy surfaces of the X 1Σg+, a 3Πu, b 3Σg-, and A 1Πu states of C2. In general, the [2]R12 and [2]S corrections enhance energy convergence for conventional multireference configuration interaction (MRCI) and multireference perturbation theory (MRMP2) calculations compared to their complete basis set limits. For the electron affinity of the F atom, [2]R12 electron affinities are within 0.001 eV of the experimental value. The [2]R12 conforme...
Highlights
Correlated R12/F12 methods1−4 are an effective approach to reduce the basis set incompleteness error (BSIE) of the conventional ab initio many-body methods
The C2 potential energy surfaces show nonparallelity errors that are within 0.7 kcal/mol compared to the complete basis set limit
The determination of electron affinity (EA) can be a difficult test for electronic structure methods because of the lack of cancellation of errors between the electron correlation and relaxation effects upon the attachment of an electron
Summary
Correlated R12/F12 methods− are an effective approach to reduce the basis set incompleteness error (BSIE) of the conventional (i.e., based on Slater determinants) ab initio many-body methods. The primary source of the BSIE is the slow convergence of the atomic basis set within Slater determinant based expansions near the singularities of the interelectronic potential. Slater determinants are computationally convenient because of factorization of n-electron matrix elements (Slater−Condon rules), they describe an interelectronic potential that is smooth everywhere, whereas the exact wave functions are known to have cusps wherever the interelectronic potential is singular. When electrons approach one another, the exact wave function is linear in the interparticle distance (rij) with the coefficient dependent on how the spins of the electrons are coupled:. An alternative is to augment the Slater determinants with basis functions that describe the cusp directly in terms of the interelectronic distances, rij, as is done in R12 methods
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