NDDO semiempirical approximations coupled with Green's function technique—a reliable approach for calculating ionization potentials

The ionization potentials of different molecules have been calculated with the outer valence Green's function (OVGF) technique, coupled with semiempirical MNDO, AM1 and PM3 methods. It is found that the OVGF method gives significantly better agreement with the experimental data than do results obtained with semiempirical calculations using Koopman's theorem including a new SAM1 and MNDO/d methods. Of the three semiempirical methods tested (MNDO, AM1, PM3) the OVGF (AM1) method gives the best agreement with experiment.