Electrostatic potential surrounding

Once you have calculated an ab initio or a semi-empirical wave function via a single point calculation, geometry optimization, molecular dynamics or vibrations, you can plot the electrostatic potential surrounding the molecule, the total electronic density, the spin density, one or more molecular orbitals /i, and the electron densities of individual orbitals You can examine orbital energies and select orbitals for plotting from an orbital energy level diagram. 

Other flexible molecular models of nitromethane were developed by Politzer et al. [131,132]. In these, parameters for classical force fields that describe intramolecular and intermolecular motion are adjusted at intervals during a condensed phase molecular dynamics simulation until experimental properties are reproduced. In their first study, these authors used quantum-mechanically calculated force constants for an isolated nitromethane molecule for the intramolecular interaction terms. Coulombic interactions were treated using partial charges centered on the nuclei of the atoms, and determined from fitting to the quantum mechanical electrostatic potential surrounding the molecule. After an equilibration trajectory in which the final temperature had been scaled to the desired value (300 K), a cluster of nine molecules was selected for a density function calculation from which... 

An insulator, on the other hand, has an ill-defined Fermi level which does not equilibrate with the spectrometer. Instead, the vacuum level of the insulator (E ) aligns with the local electrostatic potential surrounding its surface. An insulator more than a micron thick (which is the case for most catalyst samples analyzed by XPS) will not be within the local potential of the metal sample holder. The insulator will be separated from the spectrometer vacuum level (EJ) by some voltage (Vp) (30). This voltage will depend on the geometry of the sample holder and on the energy and flux of electrons from the x-ray source, the flood gun, the sample itself, and all other sources within the chamber. The potential Vp cannot be reliably measured. 

Methods to Reproduce the Molecular Electrostatic Potential (MEP). The electrostatic potential surrounding the molecule that is created by the nuclear and electronic charge distribution of the molecule is a dominant feature in molecular recognition. Williams reviews (42) methods to calculate charge models to accurately represent the MEP as calculated by ab initio methods by use of large basis sets. The choice between models (monopole, dipole, quadrapole, bond dipole, etc.. Fig. 3.12) depends on the accuracy with which one desires to reproduce the MEP. This desire has to be balanced by the increased complexity of the model and its resulting computational costs when implemented in molecular mechanics. 

Figure 20.3. The electrostatic potential surrounding the Rhodol green molecule (at 1 kfloi T/e) either in vanishing ionic strength (frame A) or in 100 mM NaCI (frame B). Please note that at high ionic strength the Coulomb cage had contracted, leaving the...

As one can see, the potential at H-bonding distances from the molecule are not too badly represented. Although more sophisticated methods to represent the electrostatic potential surrounding the molecule are clearly feasible, neither STO nor 43IG wave functions reproduce experimental polarity especially well. Thus, it is not clear that it is worth the effort to go beyond the monopole approximation since even our derived monopole potentials begin to take a non-infinitesmal amount of time on the CDC 7600 if we wish a relatively fine grid. 

---