Atomic charge model

Given the widespread use of the partial atomic charge model, it is important to consider how the charges are obtained. For simple species the atomic charges required to reproduce the... 

Cox S R and D E Williams 1981. Representation of the Molecular Electrostatic Potential by a New Atomic Charge Model. Journal of Computational Chemistry 2 304-323. 

Bellido, M. N., and J. A. C. Rullmann. 1989. Atomic Charge Models for Polypeptides Derived from Ab Initio Calculations. J. Comput. Chem. 10, 479—187. 

Lone-pairs on oxygen atoms are often included either as point charges or dipoles. This is as much an approximation as is the atomic-charge model and less effort has been directed to parameterizing this approach. 

Co SR, Williams DE (1981) Representation of the molecular electrostatic potential by a net atomic charge model. J Comput Chem 2 304-323... 

Molecular Electrostatic Potential by a Net Atomic Charge Model. 

The issue of transferability of atomic charge models between conformations has been more vigorously discussed in the last decade. Most force fields assume that the charge distribution associated with each atom is independent of conformation. However, it has been noted that potential derived atomic charges do vary with the conformation of the molecule used to derive them and that these variations have a significant effect on computed properties such as the free energy of hydration of alcohols. 

The principal reason for the latter failings lies in the absence of any representation of the lone electron pairs on the O atom. It is the presence of these lone pairs, above and below the molecular plane, that result in the correct signs of all the multipole elements and that contribute heavily to the interaction of water with an approaching molecule. There are a number of ways in which the atomic charge model can be expanded to address this problem. One possibility is to add more centers to the representation, e.g., partial negative charges above and below the molecular plane, in the approximate centers of the two lone pairs. This concept is illustrated in Fig. 2f, where overall molecular neutrality requires that 2qu + qo + 2[Pg.245]

E. Scrocco and J. Tomasi,. Adv. Quantum Chem., 11, 115 (1978), Electronic Molecular Structure Reactivity and Intermolccular Forces A Ffeuristic Interpretation by Means of Electrostatic Molecular Potentials. S. R. Cox and D. E. Williams,/. Comput. Chem., 2, 304 (1981). Representation of the Molecular Electrostatic Potential by a Net Atomic Charge Model. 

Figure 2B Electric potential difference contours for water (kj/mol). PD net atomic charge model minus the calculated potential,... 

Both rms and rrms are sensitive to the choice of grid point locations where the QM potential is evaluated. Typical values of rms and rrms are 1 to 3 kJ/mol and 3 to 9% with 6-3IG" wavefunctions. The fit generally worsens as grid points are placed closer to the molecule. In certain cases (e.g., strong lone pair effect) worse fits may be obtained with net atomic charge models. Models utilizing additional nonatomic sites, site dipoles, and/or quadrupoles usually give a much better fit. 

Table 4 shows PD net atomic charges for some oxygen-containing molecules. A net atomic charge model for water which reproduces the dipole moment does not yield the correct quadrupole moment. Additional charge sites for the water molecule will be discussed later. In the series H2O, CH3OH, (CH3)20 the... 

There are situations where a net atomic charge model does not give the desired accuracy of fit to the electric potential. Since the least-squares fitting procedure is a curve-fitting process, it is expected that addition of new variables of appropriate mathematical form will improve the fit. The addition of a new fixed site adds implicit variables, x, y, z, of the site location and an explicit variable, q, the site charge. The new site can be treated as a dummy atom with net charge q, and this charge can be optimized. 

Singh and Kollman found that a 6-31G PD net atomic charge model for water yielded rrms= 10.5%. Thus the fit is not as good as expected. Water... 

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