Electrostatic potentials approximate methods

There are several variations of this method. The PRDDO/M method is parameterized to reproduce electrostatic potentials. The PRDDO/M/FCP method uses frozen core potentials. PRDDO/M/NQ uses an approximation called not quite orthogonal orbitals in order to give efficient calculations on very large molecules. The results of these methods are fairly good overall, although bond lengths involving alkali metals tend to be somewhat in error. 

Energy, geometry, dipole moment, and the electrostatic potential all have a clear relation to experimental values. Calculated atomic charges are a different matter. There are various ways to define atomic charges. HyperChem uses Mulliken atomic charges, which are commonly used in Molecular Orbital theory. These quantities have only an approximate relation to experiment their values are sensitive to the basis set and to the method of calculation. 

The use of molecular electrostatic potentials calculated from the wave function of ligands, as has been recently suggested 50—54) seems to present a less ambiguous alternative to the more empirical approach described above. This potential can either be calculated exactly for each point of interest according to Eq. (3), or it is approximated by a suitable distribution of point charges choosen either by intuitive guess or by a less arbitrary method like the one of Hall 182>. 

The ASEP/MD method, acronym for Averaged Solvent Electrostatic Potential from Molecular Dynamics, is a theoretical method addressed at the study of solvent effects that is half-way between continuum and quantum mechanics/molecular mechanics (QM/MM) methods. As in continuum or Langevin dipole methods, the solvent perturbation is introduced into the molecular Hamiltonian through a continuous distribution function, i.e. the method uses the mean field approximation (MFA). However, this distribution function is obtained from simulations, i.e., as in QM/MM methods, ASEP/MD combines quantum mechanics (QM) in the description of the solute with molecular dynamics (MD) calculations in the description of the solvent. 

ASEP/MD, acronym for average solvent electrostatic potential obtained from molecular dynamics data, is a sequential QM/MM method that makes extensive use of the mean field approximation (MFA) [24], In solution, any static property A of the system must be calculated by averaging over the configurational space A defined by all the configurations thermally accessible to the system ... 

Deformation density maps have been used to examine the effects of hydrogen bonding on the electron distribution in molecules. In this method, the deformation density (or electrostatic potential) measured experimentally for the hydrogen-bonded molecule in the crystal is compared with that calculated theoretically for the isolated molecule. Since both the experiment and theory are concerned with small differences between large quantities, very high precision is necessary in both. In the case of the experiment, this requires very accurate diffraction intensity measurements at low temperature with good thermal motion corrections. In the case of theory, it requires a high level of ab-initio molecular orbital approximation, as discussed in Chapter 4. 

More recently, Lang and Kohn (1970) treated the jellium model with modern methods and modern computers. In particular, they included exchange terms in the frcc-clcctron approximation, as we have discussed. In lliis context it is interesting that the potential well that holds the electrons in the metal comes predominantly from theexchange potential the electrostatic potential itself gives only... 

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