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

J. Phys. Chem. A, 102 (5), 852 -860, 1998. jp9722972 S1089-5639(97)02297-4
Web Release Date: January 14, 1998

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

Charles H. Martin and Robert R. Birge*

W. M. Keck Center for Molecular Electronics and Department of Chemistry, 111 College Place, Syracuse University, Syracuse, New York 13244-4100

Received: July 15, 1997

In Final Form: November 12, 1997

Abstract:

The ab initio effective valence shell Hamiltonian (H ) theory is employed to reparametrize the standard MNDO Hamiltonian for MNDO-CI (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: U_C, U_N, _C, G_C,C, G_N,N. The _C resonance integral is adjusted to remove all electron-electron correlation from the MNDO one-electron, two-center H_u_,v -electron 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 -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 one-center, two-electron effective integrals. Subsequent calculations reproduce the lowest-lying ab initio H 20 vertical excitation energies to less than 0.2 eV on average (when full 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 achieving excellent accuracy for similar molecules when properly parametrized.

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