CNDOL: A fast and reliable method for the calculation of electronic properties of very large systems. Applications to retinal binding pocket in rhodopsin and gas phase porphine

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.