New Developments in Charge Transfer Multiplet Calculations: Projection Operators, Mixed-Spin States and π-Bonding

This paper presents a number of new additions to the charge transfer multiplet calculations as used in the calculation of L edge X-ray absorption spectra of 3d and 4d transition metal systems, both oxides and coordination compounds. The focus of the paper is on the consequences of the optimized spectral simulations for the ground state, where we make use of a recently developed projection technique. This method is also used to develop the concept of a mixed-spin ground state, i.e. a state that is a mixture of a high-spin and low-spin state due to spin-orbit coupling combined with strong covalency. The charge transfer mechanism to describe π-bonding uses the mixing of the metal-toligand charge transfer (MLCT) channel in addition to the normal CT channel and allows for the accurate simulation of π-bonding systems, for example cyanides.