Stewart charges

When doing the molecular fitting of ASA coefficients compared with promolecular ASA, the ASA coefficients no longer sum to 1 for all atoms in the molecule, because the sum of the coefficients reflects the effect of polarization. In other words, rather than having Pa = Za in promolecular ASA, the values of Pa Za in fitted ASA reflect the internal electron density redistribution. In such a way, fitted ASA coefficients are reminiscent of the Stewart atoms idea, where atoms in molecules are considered as radially distorted atoms and are connected with so-called Stewart charges. Contrary to common promolecular ASA where the atomic densities of Eq. [41] integrate to Za ... 

Flensburg, C., Larsen, S. and Stewart, R.F. (1995) Experimental charge density study ofmethylam-monium hydrogen succinate monohydrate. A salt with a very short O-H-O hydrogen bond,. /. Phys. Chem., 99, 10130-10141. 

Chen, R., Trucano, P. and Stewart, R.F. (1977) The valence-charge density of graphite, Acta Cryst., A33, 823-828. 

Fast methods for evaluating these integrals for the case of gaussian basis functions are known [12], Also, Hall has described how to get the symmetry operators (B) 1SjB, r, for any crystal space group [13]. The parameters account for thermal smearing of the charge density. In this work I use the form recommended by Stewart [14],... 

Stewart, R.F. (1991) The Application of Charge Density Research to Chemistry and Drug Design, Jeffrey, G.A. and Piniella, J.F.(eds.), Plenum Press, New York, pp. 63-101. 

One of the most useful features of a QM model is its ability to provide information about the molecular charge distribution. It is a general rule of thumb that even very low quality QM methods tend to give reasonable charge distributions. For neutral molecules, the dominant moment in the overall charge distribution is the usually dipole moment (unless symmetry renders the dipole moment zero). For a 125-molecule test set including H, C, N, O, F, Al, Si, P, S, Cl, Br, and I functionality, Stewart found mean unsigned errors in dipole moments... 

McLachlan, A. D., and Stewart, M. (1982). Periodic charge distribution in the intermediate filament proteins desmin and vimentin./. Mol. Biol. 162, 693-698. 

This interaction pattern is also reflected in the amino acid sequences of proteins such as tropomyosin, which can be shown to be composed of a basic seven-residue sequence that is repeated 40 times without interruption (Hodges et al., 1972 McLachlan and Stewart, 1975). Hydrophobic residues almost invariably occupy the second and fifth positions of the heptad and are presumably directed toward the major axis of the superhelix, where they serve to stabilize the structure by hydrophobic effects (Fig. 15). The charged side chains are also nonrandomly distributed and are believed to form interhelical ion pairs that further stabilize the structure (Talbot and Hodges, 1982). 

Kirfel, A., G. Will, and R. F. Stewart (1983). The chemical bonding in lithium metaborate, LiBOj. Charge densities and electrostatic properties. Acta. Cryst. B39, 175-85. 

But the idea that a ship could be driven by an on-board magnet and an electric charge in the ocean was not Saji s. It had been around for years, primarily the brainchild of Stewart Way, an American engineer who worked forty years in near obscurity for Westinghouse. Way had hankered as early as 1958 to develop an electric submarine without propellers or jets now in his seventies and semire-... 

Fig. 12. Total net charges in uracil calculated by different methods (from top to bottom w-HMO + o-Del Re, tt-SCF MO + a-Del Re, EHT, lEHT, CNDO/2, nonempirical). Data taken from Pullman and Pullman and from Stewart (nonempirical). Experimental net gross valence atomic populations for uracil are underlined [two sets of experimental numbers were obtained by the use of the L-shell standard STO s and L-shell SCF AO s scattering factors (in brackets), respectively]. The estimated standard deviations referring to the last decimal place are given in parentheses.

As to the direction of the dipole moments in the ground state of the molecules, two kinds of experimental measurements are available. Stewart has calculated the X-ray dipole moment of uracil within the point-charge approximation using the atomic charges for a standard STO L-shell. The calculated dipole moment is 4.0 1.3 D and its direction of 71° + 12° from N-l-C-4 toward N-3 atom. These values obtained within a point-charge model have an estimated standard error of about 30%. Nevertheless, the X-ray dipole moment is of reasonable magnitude and in close agreement with the value of 4.1 D obtained by solution measurement of uracil in dioxan (Table XX). 

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