THEORETICAL-STUDY OF THE HYPERFINE PARAMETERS OF OH

The magnetic hyperfine parameters of (OH)-O-17 are studied as a function of the one- and n-particle spaces. All of the parameters, except the spin density on oxygen, b(F)(O), are well described by modest levels of theory. Thus, we have directed much of our effort to understanding the computational requirements for accurately determining b(F)(O). Full configuration-interaction (FCI) calculations in six Gaussian basis sets give unambiguous results for the effect of uncontracting the oxygen s and p basis sets, of adding diffuse s and p functions, and of adding polarization functions to oxygen. FCI calculations in a [6s 2p 1d /2s] contracted basis give a b(F)(O) value that is in good agreement with experiment. A critical factor in any n-particle treatment is a balanced treatment of core and valence correlation. The size-extensive modified coupled-pair functional method provides b(F) values in reasonably good agreement with the FCI. Methods that account quantitatively for the valence correlation, such as complete-active-space self-consistent-field/multireference configuration interaction, appear to introduce an imbalance between core and valence contributions to b(F) when the one-particle basis sets are flexibly contracted in the core.