Aa. Voityuk et al., Extension of the neglect of diatomic differential overlap method to spectroscopy. NDDO-G parametrization and results for organic molecules, J PHYS CH A, 103(23), 1999, pp. 4553-4559
A new semiempirical scheme, referred to as NDDO-G, for calculating geometri
es and spectroscopic properties of molecules, is described. The method is b
ased on the NDDO (neglect of diatomic differential overlap) approximation.
It uses the point-charge model and the Mataga-Nishimoto formula to evaluate
two-center two-electron integrals. The NDDO-G model has been parametrized
for the elements H, C, N, and O. Molecular geometries of organic molecules
are well predicted by NDDO-G; for 60 molecules, the mean absolute error of
bond lengths is 0.014 Angstrom and that of bond angles is 1.9 degrees. The
spectroscopic variant of the NDDO-G scheme provides electronic excitations
using configuration interaction of singly excited states (CIS). It has been
applied to calculate absorption spectra (vertical transitions) of several
dozens organic molecules and photoelectron spectra within Koopmans' approxi
mation. These NDDO-G results are compared with experimental results and wit
h results of high-level ab initio calculations. The mean absolute error of
NDDO-G excitation energies is 0.13 eV = 1050 cm(-1) (196 comparisons). Firs
t and several higher ionization potentials are reproduced with a mean absol
ute error of 0.24 eV (123 comparisons). The proposed method may be used for
studying structures of large organic and biological molecules and for inte
rpreting and predicting their absorption and photoelectron spectra. As an e
xample, we discuss the spectroscopy of free-base porphin.