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, cyclobuta diene, and trans-butadiene in order to examine the accuracy of approxi mate H-v calculations specifically designed to reproduce a Pariser-Par r-Pople-like (PPP) semiempirical model from first principles. This is the first H-v study in which systematic, semiempirical-like approximat ions to H-v are examined for their accuracy and basis set dependence i n computations of vertical excitation energies and ionization and pote ntials for large, polyatomic molecules. We focus explicitly on two fun damental semiempirical-like approximations to ab initio H-v calculatio ns. First, it is shown that ab initio H-v calculations can produce rea sonably accurate vertical excitation energies and ionization potential s for the low-lying valencelike states when using valence orbitals ''c onstrained'' to be linear combinations of transferable, carbon atom p( pi) orbitals. The differences between full ab initio H-v and constrain ed 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 sta tes and, furthermore, these constrained errors show no basis set depen dence. Second, it is shown that the excitation energies and ionization potentials are not influenced significantly by the nonclassical H-v t hree- and four-body operators. When neglecting these interactions, err ors of only a few tenths of an electron volt are again encountered, an d 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 compare d directly with the one- and two-body empirical parameters which appea r in the PPP-like model. The present calculations explain the errors e xpected to occur in further, approximate H-v calculations, the states which can be reasonably described with both approximate H-v calculatio ns and PPP-like models, and the ab initio basis set requirements;For f uture ab initio H-v studies of these models.