EOM-CCSD calculation of metal K pre-edge spectra: 3d transition metal tetrachlorides

The metal K pre-edge spectra of 3d transition metal tetrachlorides (MCl4, M = Ti, Fe, Co, Ni, and Cu) are obtained using the equation of motion coupled cluster (EOM-CC) approach. These spectra are primarily influenced by two key contributions to the oscillator strength-the electric dipole and quadrupole transition moments-due to the possible mixing of 3d and 4p orbitals in transition metal atoms. The EOM-CC singles and doubles method incorporating a recently implemented formalism that includes all the second-order contributions to oscillator strength [i.e., beyond the customary dipole approximation, Park et al., J. Chem. Phys. 155, 094103 (2021)] provides a powerful tool for computing excitation energies and oscillator strengths. This approach enables accurate interpretation of experimental spectra and facilitates predictions when experimental data are unavailable. In the present study, we demonstrate how these new extensions to the EOM-CC method can be utilized to compute metal K pre-edge spectra and determine the orbital characteristics of MCl4 complexes.