The first excited triplet states of the nucleotide bases calculated with different semiempirical SCF schemes

The π-electron distributions, spin densities, and energies of the first triplets of the nucleotide bases, uracil, thymine, cytosine, adenine, and guanine, were investigated in various semiempirical approximations. Results are presented for calculations using the semiempirical form of the closed-shell SCF configuration interaction method, of the different orbitals for different spins (unrestricted Hartree–Fock) treatment, with and without spin projection, and of the Roothaan's open-shell procedure.