A new nonperturbative theory of core-hole ionizations: a compact cluster-expansion technique for treating relaxation effects

Abstract

We have introduced in this paper a new cluster-expansion ansatz for the wave-operator Ω that incorporates orbital relaxation and differential correlation effects in a compact and efficient manner. The ansatz utilizes a Thouless-type transformation inducing these effects via a seminormal ordered exponential-type cluster operator, denoted as exp_r[T+S^c+S^r], with T, S^c and S^r as the core-correlation, valence-correlation and relaxation-cum-differential-correlation cluster operators, respectively. The theory is illustrated by using the example of one-valence core ionizations. Numerical applications illustrate both the superiority of the method to the normal-ordered ansatz for Ω, and the relative unimportance of S^c for this problem.