Accuracy atomic charges

Although cross terms between the bonded potentials are part of all force fields designed to aclfieve high accuracy, the coupling between the geometry and the atomic charges is rarely addressed. From electronic structure calculations it is known that the optimum set... 

Another problem of atomic charges determined by fitting is related to the absolute accuracy. Although inclusion of charges on all atoms does not significantly improve the results over those determined from a reduced set of parameters, the absolute deviation between the true and fitted electrostatic potentials can be quite large. Interaction... 

Given the diversity of different SCRF models, and the fact that solvation energies in water may range from a few kcal/mol for say ethane to perhaps 100 kcal/mol for an ion, it is difficult to evaluate just how accurately continuum methods may in principle be able to represent solvation. It seems clear, however, that molecular shaped cavities must be employed, the electiostatic polarization needs a description either in terms of atomic charges or quite high-order multipoles, and cavity and dispersion terms must be included. Properly parameterized, such models appear to be able to give absolute values with an accuracy of a few kcal/mol." Molecular properties are in many cases also sensitive to the environment, but a detailed discussion of this is outside the scope of this book. ... 

Indeed, both expressions predict quadratic dependence of AA on the dipole moment of the solute. As in the previous example, it is of interest to test whether this prediction is correct. Such a test was carried out by calculating AA for a series of model solutes immersed in water at different distances from the water-hexane interface [11]. The solutes were constructed by scaling the atomic charges and, consequently, the dipole moment of a nearly spherical molecule, CH3F, by a parameter A, which varied between 0 and 1.2. The results at two positions - deep in the water phase and at the interface - are shown in Fig. 2.3. As can be seen from the linear dependence of A A on p2, the accuracy of the second-order perturbation theory... 

Table 3.29. The NBO descriptors of XYZ triatomic anions (see Table 3.28), showing natural atomic charges (Q), percentage polarization of oxy and ayz NBOs toward terminal atoms, second-order nz— oxy and nx->OYZ stabilizations (A/s(2)), and occupancies of valence antibonds (oxy and ayz ) and extra-valent Rydberg orbitals (ry ) parenthesized values refer to Lewis structure of lower accuracy...

The contribution to the predicted electrostatic potential of the anisotropic atomic multipoles (Q , / > 0), which represent the lone pair and n-electron density, rapidly become less important as the distance between the molecules increases. This not only results from the inverse power of R increasing with I, but also from the cancellation between the contributions from different multipoles and different atoms. For example, there is generally an atomic dipole component along a bond that opposes the polarity implied by the atomic charges, as shown in the results of distributed multipole analyses (DMAs) of the azabenzenes. ° Thus, the accuracy gained by using a distributed multipole model is very dependent on the relative separation and orientation of the molecules, as well as the actual distribution of charge in the molecule. 

There have been many studies that contrast the accuracy of various atomic charge and distributed multipole models. These studies include the extensive tests provided when various distributed multipole methods were first proposed. For example, there are published contour plots of the potential around a water molecule, the amino acid histidine, and variations in the electrostatic energies of nucleic acid bases,which confirm the significance of the atomic anisotropy shown in the color three-dimensional displays of the electrostatic field around uracil and pyrimidine. It is clear that the difference... 

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