Double-core-hole (DCH) spectra have been investigated for pyrimidine, purine, the RNA/DNA nucleobases, and formamide, using the density functional theory (DFT) method. DCH spectra of formamide were also examined by the complete-active-space self-consistent-field (CASSCF) method. All possible single- and two-site DCH (ssDCH and tsDCH) states of the nucleobases were calculated. The generalized relaxation energy and interatomic generalized relaxation energy were evaluated from the energy differences between ssDCH and single-core-hole (SCH) states and between tsDCH and SCH states, respectively. The generalized relaxation energy is correlated to natural bond orbital charge, whereas the interatomic generalized relaxation energy is correlated to the interatomic distance between the core holes at two sites. The present analysis using DCH spectroscopy demonstrates that the method is useful for the chemical analysis of large molecular systems.
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