Ab initiocomputation of semiempirical π-electron methods. I. Constrained, transferable valence spaces in ℋν calculations

Publisher Website
Google Scholar
Abstract
Large basis set Hν calculations are performed for ethylene, cyclobutadiene, and trans‐butadiene in order to examine the accuracy of approximate Hν calculations specifically designed to reproduce a Pariser–Parr–Pople‐like (PPP) semiempirical model from first principles. This is the first Hν study in which systematic, semiempirical‐like approximations to Hν 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 Hν calculations. First, it is shown that ab initio Hν 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 pπ orbitals. The differences between full ab initio Hν and constrained Hν 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 Hν 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 Hν calculations. Because the three‐ and four‐body Hν terms are neglected as a leading approximation, the computed oneand two‐body empirical parameters which appear in the PPP‐like model. The present calculations explain the errors expected to occur in further, approximate Hν calculations, the states which can be reasonably described with both approximate Hν calculations and PPP‐like models, and the ab initio basis set requirements for future ab initio Hν studies of these models.

This publication has 43 references indexed in Scilit: