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 (A(v)) theory is employed to reparametrize the standard MNDO Hamiltonian for MNDO-pi CI (configuration interaction) excited-state calculations for the small, protonated Schiff base 2,4-pentadien-1-iminium, The revised parametrization, called H-MNDO(pi), differs from the ground-state parametrization via adjustment of the basic p-orbital parameters: U-C, U-N, beta(C) G(C,C), G(N-N). The beta(C) resonance integral is adjusted to remove all electron-electron correlation from the MNDO one-electron, two-center H-u,H-v pi-eleclron integrals. Likewise, a small correction of 0.25 eV is appended to both U-C and U-N. The MNDO ground-state G(C,C) and G(N,N) pi-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 A(v) one-center, two-electron effective integrals. Subsequent H-MNDO(pi), calculations reproduce the lowest-lying ab initio H-v 20 vertical excitation energies to less than 0.2 eV on average (when full pi CI is employed). We also estimate the size-consistency errors in previous ab initio MRSDCI calculations for the planar and twisted geometries to be as large as 0.2-0.3 eV. Thus, the MNDO-CI method has the potential of athieving excellent accuracy for similar molecules when properly parametrized.