From Density Functional Theory to Density Matrix Functional Theory

Coupled cluster methods are considered among the most accurate tools in electronic structure theory. Nonetheless relatively limited attempt seems to have been made to extend their applicability to the description of the core-excitation phenomena that are behind largely used spectroscopic techniques like x-ray absorption spectroscopy and x-ray circular dichroism. As a first step to redeem for such situation, we present an implementation of damped response theory (aka complex polarization propagator approach) based on an asymmetric Lanczos algorithm for the CCS, CC2 and CCSD hierarchy of coupled cluster methods, and apply it to the simulation of the K-edge x-ray absorption spectra of various closed-shell systems, including Neon, CH4, H2O, HF and CO [1]. Triple excitation effects on the excitation energies are estimated by means of the CCSDR(3) approximation, and relativistic effects are accounted for using the Douglas-Kroll approach. Results are compared with experiment as well as results obtained with other computational methods. [1] S. Coriani, T. Fransson, O. Christiansen, P. Norman, P. Decleva. Work in progress.