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G.N.Lewis' Model

This has become the largest part of this site. I have chosen to render G.N.Lewis' molecular formulas into a quantitative tool. With the simplest representation of the nuclear-electronic structure, using G.E. Kimball's free [spherical electron probability] clouds. I show that one can obtain ground state energy and energy components with a quality of small basis sets (~ HF/6-31G*) in seconds to minutes for molecules of some 30 to several 1000 atoms. E.g. the small protein Crambin with 644 atoms (46 aminoacid residues) takes 30 sec, the Mammalian Cytochrome p450 2B4 with 8329 atoms 45 min on a standard desktop PC (~450 examples).

Kimball's free [spherical electron probability] clouds give only a crude electronic charge distribution in a molecule. They cannot simulate the subtle details of local electronic density variations, often decisive for the path of chemical reactions. However, the model gives a reliable overall description of ground states. Properly optimized structures have Vne and Vee terms close to elaborate QC computations. I.e. the gross charge distribution is adequate. The model is validated by obeying the three fundamental laws of the chemical world: Hellmann-Feynman electrostatic theorem, Virial theorem, and Pauli's exclusion principle. Many popular QC methods with very crude approximations of molecular orbitals do not fulfill any or only some of these laws: e.g. Heitler-London's H2, LCAO-MO and VB schemes in zeroth order, and most semiempirical methods. Advanced QC methods offer structure optimizations which lead to very small residual forces on the nuclei. Hence, they are of value for understanding chemical bonding.

If you compute the ground state of a molecule but allow nuclei in their electronic charge distribution to violate an electrostatic force equilibrium you produce an example of a perpetuum mobile: Such a molecule is in non-thermally excited vibrational states, hence vibrates continuously and thus may perform work and radiate infrared light. Or, less dramatically: You have failed to compute the ground state because your program uses an incomplete description of the physical system!
These pages offer ample proof of these statements. Begin with Tutorials!