Molecular calculations with the MODPOT, VRDDO, and MODPOT/VRDDO procedures. II. Cyclopentadiene, benzene, diazoles, diazines, and benzonitrile

Abstract

A procedure for carrying out nonempirical LCAO–MO–SCF calculations on large molecules is outlined. Use is made of an approximation for neglecting some one‐ and two‐electron integrals proposed by Wilhite and Euwema. The differential overlap between a pair of basis functions is set to zero if the pseudo‐overlap is less than the adjustable threshold. We refer to this method as variable retention of diatomic differential overlap (VRDDO). In addition, only the valence electrons may be explicitly considered. The influence of the core electrons is taken into account by using the model potential (MODPOT) Hamiltonian suggested by Bonifacic and Huzinaga. The two methods can be used together resulting in the MODPOT/VRDDO procedure. VRDDO, MODPOT, and MODPOT/VRDDO calculations are reported for cyclopentadiene, benzene, pyrazole, imidazole, pyridazine, pyrimidine, pyrazine, and benzonitrile. The accuracy of the VRDDO calculations is very good. Orbital energies and gross atomic populations differ from the reference values by only 0.001–0.002 a.u. The maximum error in the total energy is 0.0024 a.u. and in the dipole moment is 0.009 a.u. Valence electron‐only calculations with the MODPOT and MODPOT/VRDDO procedures yield orbital energies and charge densities that the quite good in comparison with the ab initio calculations taking all electrons into consideration. Orbital energies and gross atomic populations differ from the reference values b by 0.001–0.011 a.u. The maximum error in the dipole moment is 0.018 a.u. For benzonitrile, the largest molecule studied, introduction of the VRDDO procedure results in a 25% saving in computer time. The MODPOT calculation is three times faster than the ab initio reference calculation, while the MODPOT/VRDDO calculation is four times faster.