A complete formalism is presented for the molecular-orbital study of a solute in a continuum. The Schrödinger equation of the solute in the solvent is derived from that for the entire solute–solvent system. An arbitrarily shaped cavity boundary is constructed using finite element techniques based on hexagonal and pentagonal surface elements and the induced charge on its surface calculated using analytical formulas for the electrostatic field strength. The Fock operator, which differs from one in widespread use, is modified by two terms resulting from variations in both the electrostatic field of the solute and the induced charges. An Austin Model 1 (AM1) version of the theory is developed with the addition of no new semiempirical parameters and illustrated with calculations on dimethyl ether and propane.
Atomic charges derived from a recently described approach to the very rapid computation of AM1 electrostatic potentials (ESP) accurately parallel, but are ca. 20% smaller than, the corresponding HF/6-31G* values. The dipole moments computed from the AM1 charges are virtually identical to those derived directly from the wave function and in rather better agreement with the experimental values than those computed using the HF/6-31G* charges. Unlike other approaches to the semiempirical calculation of ESP-derived charges, the present method also yields near HF/6-31G* quality potentials close to the molecular periphery. For medium-sized organic molecules (40-100 basis functions), the method is approximately two orders of magnitude faster than those involving prior deorthogonalization of AM1 wave function and explicit computation of the full ESP integral matrix.& Sons, Inc. 0 1994 by John Wiley cepted as the most appropriate for the definition of intermolecular potential functions, especially those used in dynamic simulations.1°-12 To a considerable extent, the success of these procedures depends on the speed and accuracy with which the ESP and/or ESP-derived charges can be obtained.The ab inifio calculation of the ESP, defined by eq.(1) in terms of the Hartree-Fock (HF) wave function, is straightforward but rapidly becomes extremely expensive as the size of the molecule increases. This effectively precludes its use in just
A new approach to the computation of molecular electrostatic potentials based on the AM1 wave function is described. In contrast to the prevailing philosophy, but consistent with the underlying NDDO approximation, no deorthogonalization of the wave function is carried out. The integrals required for the computation of the electronic contributions to the molecular electrostatic potential are evaluated in a manner similar to that of the AM1 core-electron attraction integrals, while the nuclear contributions are computed using a new semi-
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