Reparametrizing MNDO for Excited-State Calculations by Using ab Initio Effective Hamiltonian Theory: Application to the 2,4-Pentadien-1-iminium Cation

The ab initio effective valence shell Hamiltonian (ℋ ν) theory is employed to reparametrize the standard MNDO Hamiltonian for MNDO-πCI (configuration interaction) excited-state calculations for the small, protonated Schiff base 2,4-pentadien-1-iminium. The revised parametrization, called differs from the ground-state parametrization via adjustment of the basic p-orbital parameters:  UC, UN, βC, GC,C, GN,N. The βC resonance integral is adjusted to remove all electron−electron correlation from the MNDO one-electron, two-center Hu,v π-electron integrals. Likewise, a small correction of 0.25 eV is appended to both UC and UN. The MNDO ground-state GC,C and GN,N π-electron one-center, two-electron repulsion integrals are increased in value by about 1.5 and 2 eV, respectively, to reproduce the average of the ab initio ℋ ν one-center, two-electron effective integrals. Subsequent calculations reproduce the lowest-lying ab initio ℋ ν 20 vertical excitation energies to less than 0.2 eV on average (when full πCI is e...