HF-STEX and RASSCF calculations on nitrogen K-shell X-ray absorption of purine base and its derivative.

The nitrogen K-shell X-ray absorption spectra of the purine bases present in nucleic acids, adenine and guanine, were analyzed by using ab initio Hartree-Fock static exchange and restricted-active-space self-consistent-field calculations. A variety of derivative molecules were calculated to investigate the energetic shifts due to environmental effects on the nitrogen atoms. Shake-up excitations were also addressed.