Study of influences of various excitation classes onab initio calculated isotropic hyperfine coupling constants

Summary Reliable prediction of the isotropic hyperfine coupling constantA _iso is still a difficult task forab initio calculations. Strong dependence on the method employed for its calculation has been found. Within a CI ansatzA _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 onA _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.