Abinitio computation of semiempirical π‐electron methods. I. Constrained, transferable valence spaces in Hν calculations

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 except...