STUDY OF INFLUENCES OF VARIOUS EXCITATION CLASSES ON AB-INITIO CALCULATED ISOTROPIC HYPERFINE COUPLING-CONSTANTS

Reliable prediction of the isotropic hyperfine coupling constant A(iso) is still a difficult task for ab initio calculations. Strong dependence on the method employed for its calculation has been found. Within a CI ansatz A(iso) is considerably affected by the excitation classes taken into account within the CI calculation. In the present work the influence of various excitation classes on A(iso) is examined. Calculations including all single, double, triple and a large part of the quadruple excitations are performed and the individual effects of the excitation classes are studied. It is found that the surprisingly good agreement found for S-CI treatments is due to large error cancellations. The importance of higher than double excitations arises from their indirect influence on the single excitations.