ESTIMATION OF THE INFLUENCE OF THE CONFIGURATIONS NEGLECTED WITHIN TRUNCATED MULTIREFERENCE CI WAVE-FUNCTIONS ON MOLECULAR-PROPERTIES

Reliable prediction of the isotropic hyperfine coupling constant, a(iso), is still a difficult task for ab initio calculations. Strong dependence on the method used for its calculation is found. Within a truncated multi-reference ansatz a(iso) is strongly affected by the size of the reference space and the number of terms in the truncated CI expansion. In the present paper different effects of the neglected CI space are discussed. Modified B(K) and A(K) methods are used to estimate the contributions of the neglected configurations. It can be shown that a combination of both methods is able to recover about 90-95% of the total error in a(iso). Furthermore, it was found that to obtain about 90% of the B(K) correction only about 10-20% of the configurations within H0 have to be corrected.