Ionization energies of azines from green's function method in semiempirical AM1 approximation

Abstract

The ionization energies of a number of azines have been calculated using the OVGF method in the semiempirical AM1 approximation. The SCF AM1 and MNDO calculations fail to reproduce correctly the ordering of the ionization energies for the azines under consideration. The quantitative agreement of the OVGP(AM1) values with the observed photoelectron spectra is at the same level as that of the OVGF ab initio calculation with a polarized basis set. The computer time needed for the latter is 40–50 times that of the OVGF (AM1) calculation. For all the molecules under consideration the OVGF (AM1) method gives much better quantitative agreement with experiment than does the semiempirical HAM/3 method. In the particular case of the pyridine molecule, the OVGF (AM1) method reproduces correctly the degeneration of the two upper occupied n- and π-type levels, which is in contrast with the HAM/3 method.