Gerdy, James Joseph (1996) Accurate interatomic potentials for simulations. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:10212009-150813700
This thesis develops a rational foundation for the application of long-range forces to atomistic simulations. This area has lagged behind the other components of simulations because of two factors. First, the nonbond forces are difficult to probe experimentally. There are only a few materials for which properties clearly and directly correlate with van der Waals forces (such as molecular crystals) and then some of these cases are not relevant to common modeling applications (e.g. the hydrogen van der Waals forces in H_(2(xtl)) to are different from those in hydrocarbons). Second, more than for valence force field terms, van der Waals forces are difficult to determine by calculation. The forces are weak and require a large number of basis functions per atom to treat properly. This thesis contains a method which has optimized a level of ab initio calculation on small clusters in order to extract a van der Waals potential. The size of the calculation is controlled by carefully optimizing the basis set. Moreover, unlike for previous calculations of this sort, it was recognized that the repulsive and attractive potentials (the monotonic potentials) which constitute the van der Waals potential can be calculated optimally with different basis sets, further accelerating the calculation for a given level of accuracy. Also the use of diffuse basis sets off of the atom centers is used here to make the basis sets more efficient. The method has been optimized for the case of nitrogen because it is both a closed shell case and relatively common in simulations. What results is a computational method which produces pair potentials for use in force field simulations. This is called the combination of monotonic potentials (COMP) method. Subsequently, potentials have been calculated for the atoms H, He, C, N, 0, F, Ne, Si, P, S, Cl, and Ar. In order to test the accuracy, the potentials are applied to test cases of molecular crystals and compared to other commonly used potentials. Important issues that are addressed are standard combination rules and the accuracy of using isotropic pair potentials. COMP potentials give a measure of accuracy of van der Waals potentials for any atom. This research has also yielded an accurate functional form, a variant of the Morse potential, which has not been used in simulations but provides very accurate fits to the ab initio data. The relationships between different functional forms are analyzed so that the designer of force fields can make a judicious choice of both ab initio calculations to determine potentials and the appropriate functions with which to fit them.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||1 December 1995|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||17 Nov 2009 19:27|
|Last Modified:||26 Dec 2012 03:18|
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