Ab initio calculations on large molecules using molecular fragments. SCF-MO-CI Studies on low-lying singlet and triplet states of pyrazine

Wave functions for the ground state and low-lying 1,3(n→*) and 1,3(π→π*) excited states of pyrazine are obtained from a configuration interaction procedure based on molecular orbitals from a ground-state SCF calculation. A detailed comparison is made between the present results and both experimental data and more extensive calculations, indicating that the qualitative ordering of excited states predicted here is substantially correct. Expansion coefficients of the principal configurations for most excited states resemble closely those of more accurate calculations, and for excited states where Franck-Condon transition energies are known, a linear relationship between experimental and computed transition energy values is obtained. These observations, as well as the favorable computational characteristics of the molecular fragment approach, indicate that the techniques used on pyrazine are well suited for studies of the spectra of large molecules.