  In this paper, we study energies and $g$-factors for a large number of levels
of Th I. We found that the accuracy of CI-MBPT can be substantially improved by
using multiple adjustment parameters, which are introduced to regulate the
second-order MBPT corrections for single valence energies and Coulomb
interaction. The results for energies and $g$-factors are in excellent
agreement with experiment, with accuracy sufficient to positively assign
theoretical states of given angular momentum and parity to experimental levels.
This theory will be further developed in the future to treat other, even more
complicated actinides such as U I.
