Electronic structures of the ground state and excited singlet states of nucleic acid bases

The self-consistent field (SCF) method is used in the semiempirical approximation (SA) with allowance for singly excited configurations to derive the ground state and singlet excited states for five nucleic acid bases (NAB). Empirical values are used for the nondiagonal elements of the matrix for the two-electron Coulomb integrals. The methyl group in thymine is allowed for via an induction model. Theoretical values are given for the energies of the first four electronic transitions for each base. The locations of these agree well with spectroscopic data. The energies of the first two transitions in adenine are found to be very similar when allowance is made for configuration interaction, so the first absorption band of adenine arises from overlap of two transitions.