Using effective operators in calculating the hyperfine structure of atoms

We propose a method for calculating the hyperfine structure (hfs) of multie lectron atoms based on a combination of configuration superposition and man y-body perturbation theory. The latter is used to construct an effective Ha miltonian and an effective hfs operator in configurational space. The metho d can be applied in calculations of the matrix elements of any one-electron operators. By way of an example we calculate the magnetic hfs constant A f or several lowest levels of neutral thallium. We show that the method achie ves a calculation accuracy of about 1%, which earlier was possible only for atoms with a single valence electron. (C) 1998 American Institute of Physi cs. [S1063-7761(98)00611-8].