AB-INITIO COMPUTATION OF SEMIEMPIRICAL PI-ELECTRON METHODS .1. CONSTRAINED, TRANSFERABLE VALENCE SPACES IN H-V CALCULATIONS

Large basis set H-v calculations are performed for ethylene, cyclobutadiene, and trans-butadiene in order to examine the accuracy of approximate H-v calculations specifically designed to reproduce a Pariser-Parr-Pople-like (PPP) semiempirical model from first principles. This is the first H-v study in which systematic, semiempirical-like approximations to H-v are examined for their accuracy and basis set dependence in computations of vertical excitation energies and ionization and potentials for large, polyatomic molecules. We focus explicitly on two fundamental semiempirical-like approximations to ab initio H-v calculations. First, it is shown that ab initio H-v calculations can produce reasonably accurate vertical excitation energies and ionization potentials for the low-lying valencelike states when using valence orbitals ''constrained'' to be linear combinations of transferable, carbon atom p(pi) orbitals. The differences between full ab initio H-v and constrained H-v calculations for all states (with one exception each in trans- and cyclobutadiene) range between roughly 0.00 and 0.25 eV for all states and, furthermore, these constrained errors show no basis set dependence. Second, it is shown that the excitation energies and ionization potentials are not influenced significantly by the nonclassical H-v three- and four-body operators. When neglecting these interactions, errors of only a few tenths of an electron volt are again encountered, and these errors also display no basis set dependence. These conclusions are shown to be consistent with previous ab initio H-v calculations. Because the three- and four-body H-v terms are neglected as a leading approximation, the computed one- and two-body H-v terms can be compared directly with the one- and two-body empirical parameters which appear in the PPP-like model. The present calculations explain the errors expected to occur in further, approximate H-v calculations, the states which can be reasonably described with both approximate H-v calculations and PPP-like models, and the ab initio basis set requirements;For future ab initio H-v studies of these models.