Nonrelativistic ab initio calculation

Thus, the accuracy of a (nonrelativistic) ab initio calculation is determined by two factors, i.e. the size and quality of the basis set and the method by which correlation energy has been calculated. The accuracy can systematically be improved by choosing better basis sets and better methods for calculating correlation energy. 

The mean-field approximation requires the proper occupation numbers of the valence orbitals (w = 2 for core and 111 = 0 for virtual molecular orbitals). These should be taken from a nonrelativistic ab initio calculation. Nevertheless, all reasonable choices proved to be suitable for practical purposes because they cause only negligible fluctuations in the calculated energy of spin-orbit coupling. 

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