Ab-Initio Multiplet Calculations Using Iterative Algorithms for X-ray Absorption Spectra at Transition Metal L_2,3-Edges

Abstract

X-ray absorption spectra at the L_2,3-edges of transition metals show widely spreading multiplet structure due to the strong electronic correlations between the 2p and 3d electrons. The ab-initio multiplet method based on the relativistic configuration interaction (CI) theory is one of the most reliable theoretical methods to reproduce and predict such spectra. In this method, a many-electron Hamiltonian matrix is fully diagonalized in order to obtain the many-electron wavefunctions for the initial and final states of transitions simultaneously. Since the dimension of the Hamiltonian matrix grows exponentially with an increase in the number of active orbitals, the method has been only computationally feasible only for small systems. In the present study, iterative algorithms, including the Davidson-like algorithm for obtaining the wavefunctions for the initial states, and Lanczos algorithm for evaluating the spectral functions solely from the initial states, were implemented. The theoretical spectra obtained by the new algorithms are identical to those obtained by the full-diagonalization method. As the iterative algorithms adopted in this study require much less memory space, the ab-initio multiplet method with iterative algorithms can be applied to larger systems that are unmanageable by a conventional full-diagonalization method.