Coupled cluster study of the x-ray absorption spectra of formaldehyde derivatives at the oxygen, carbon, and fluorine K-edges.

We have investigated the performance of a core-valence separated scheme within the coupled cluster (CC) hierarchy of methods CC singles (CCS), CC singles and approximate doubles (CC2), and CC singles and doubles (CCSD) in reproducing the K-edge x-ray absorption spectra of the low-Z elements carbon, oxygen, and fluorine in formaldehyde (CH₂O), carbonyl fluoride (CF₂O), formyl fluoride (CHFO), and formic acid (CHOOH). The analysis covers the entire frequency region from the first 1s → π^* excitation to the core-ionization limit, encompassing the region of Rydberg transitions. Moreover, a simulation of the vibronic progressions in the 1s → π^* bands of both carbon and oxygen in formaldehyde has been performed at the core-valence separated CCSD level, and the results are critically compared with highly resolved experimental data for this molecule. [ABSTRACT FROM AUTHOR]