Very large mol. systems can be calcd. with the so called CNDOL approx. Hamiltonians that have been developed by avoiding oversimplifications and only using a priori parameters and formulas from the simpler NDO methods. A new diagonal monoelectronic term named CNDOL/21 shows great consistency and easier SCF convergence when used together with an appropriate function for charge repulsion energies that is derived from traditional formulas. It is possible to obtain a priori MOs and electron excitation properties after the CI of single excited determinants with reliability, maintaining interpretative possibilities even being a simplified Hamiltonian. Tests with some unequivocal gas phase maxima of simple mols. (benzene, furfural, acetaldehyde, hexyl alc., Me amine, 2,5 di-Me 2,4 hexadiene, and Et sulfide) ratify the general quality of this approach in comparison with other methods. The calcn. of large systems as porphine in gas phase and a model of the complete retinal binding pocket in rhodopsin with 622 basis functions on 280 atoms at the quantum mech. level show reliability leading to a resulting first allowed transition in 483 nm, very similar to the known exptl. value of 500 nm of "dark state. " In this very important case, our model gives a central role in this excitation to a charge transfer from the neighboring Glu- counterion to the retinaldehyde polyene chain. Tests with gas phase maxima of some important mols. corroborate the reliability of CNDOL/2 Hamiltonians.