Charges fitting electrostatic potential

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... 

An improved method was developed by Chirlian and Francl and called CHELP (CHarges from ELectrostatic Potentials). Their method, which uses a Lagrangian multiplier method for fitting the atomic charges, is fast and noniterative and avoids the initial guess required in the standard least-squares methods. In this approach, the best least-squares fit is obtained by minimizing y ... 

The fitting is done by minimizing an error function of the form shown in eq. (2.23), under the constraint that the sum of the partial charges (2, is equal to the total molecular charge. The electrostatic potential is sampled at a few thousand points in the near vicinity of the molecule. 

An alternative approach [24, 55, 56] of H-bonding ranking is based on COSMO-RS fluid thermodynamics theory [57], Abramov [24, 58] performed a study of the correlation of two independent schemes of atomic charge calculations (polarization a H-bonding charges and electrostatic potential (ESP) fitted charges, with Abraham s experimental H-bonding scales and available... 

Waals interaction and a Coulombic term for electrostatic interactions, are used for interactions between pairs of atoms that are separated by three or more intervening atoms, or those that belong to different molecules. The treatment of electrostatic interactions is based on ab initio charges derived by fitting electrostatic potentials with constraints. 

Electrostatic potential-derived charges assign point charges to fit the computed electrostatic potential at a number of points on or near the van der Waals surface. This sort of analysis is commonly used to create input charges for molecular mechanics calculation. 

Fitting atomic charges to give a best match to a calculated electrostatic potential. The constraint is that the sum of atomic charges should equal the net charge of the molecule. 

A classical description of M can for example be a standard force field with (partial) atomic charges, while a quantum description involves calculation of the electronic wave function. The latter may be either a semi-empirical model, such as AMI or PM3, or any of the ab initio methods, i.e. HF, MCSCF, CISD, MP2 etc. Although the electrostatic potential can be derived directly from the electronic wave function, it is usually fitted to a set of atomic charges or multipoles, as discussed in Section 9.2, which then are used in the actual solvent model. 

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