Comparison of INDO and Ab Initio Methods for Correlated Wave Functions of the Ground and Excited States of Methylene and Ethylene

The usefulness of the INDO integral approximation for correlated wave functions was tested by carrying out GVB calculations on (1) the three lowest states of methylene as a function of bond angle and (2) the three lowest states of ethylene as a function of the dihedral twist angle. The methylene potential curves obtained with INDO were in good agreement (0.2 eV errors) with ab initio results, while the ethylene curves were very poor (2 to 4 eV errors). Comparison with ab initio calculations revealed two major problems in the INDO method: (1) the use of empirical values from atomic spectra for the one-center exchange integrals and (2) the use of only one resonance or β parameter per atom.