Highly Accurate ab Initio π-Electron Hamiltonians for Small Protonated Schiff Bases

The effective valence shell Hamiltonian (ℋ ν ) method is used to compute the ab initio π-electron Hamiltonians for the two small protonated schiff bases propeniminium (C 3 H 4 NH 2 + ) and pentaniminium (C 5 H 6 NH 2 + ), which are small analogs of the photoreceptor chromophore retinal. Because these small polyenes lack low lying, ionic excited states, the ℋ ν calculations perform remarkably well even though they employ constrained, semiempirical-like valence spaces. In contrast to studies on other polyenes, even the second-order ℋ ν performs extremely well, while the third-order ℋ ν calculations yield an effective π-electron Hamiltonian that resembles the familiar semiempirical Pariser−Parr−Pople (PPP) Hamiltonian but which also retains ab initio accuracy for the vertical excitation energies. The ab initio π-electron Hamiltonian need only retain the PPP-type α i, β i, j, and γ i, j effective integrals to produce excitation energies that compare well with previous multireference single and double configuration interaction (MRSDCI) calculations. Since the ab initio π-electron effective integrals arise from first principles, they do not require any empirical corrections. However, they do lack the simple transferability generally assumed in semiempirical models for these short-chain polyenes.