The effective valence shell Hamiltonian (H) method is used to compute the ab initio pi-electron Hamiltonians for the two small protonated schiff bases propeniminium (C3H4NH24) and pentaniminium (C5H6NH24), which are small analogs of the photoreceptor chromophore retinal. Because these small polyenes lack low lying, ionic excited states, the H 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 performs extremely well, while the third-order H calculations yield an effective pi-electron Hamiltonian that resembles the familiar semiempirical Pariser-Parr-Pople (PPP) Hamiltonian but which also retains nb initio accuracy for the vertical excitation energies. The ab initio pi-electron Hamiltonian need only retain the PPP-type alpha(i), beta(ij), and gamma(ij) effective integrals to produce excitation energies that compare well with previous multireference single and double configuration interaction (MRSDCI) calculations. Since the ab initio pi-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.