Force-field techniques

Approximating the intermoleculai interactions to only include two-body effects, e.g. electrostatic forces are only calculated between pairs of fixed atomic chai ges in force field techniques. Or the discrete interactions between molecules may be treated only in an average fashion, by using Langevin dynamics instead of molecular dynamics. 

Although IR frequencies provide a useful measure of the extent of v bonding in carbonyl complexes, a better quantitative picture can be obtained from C—O force constants. These values are commonly derived from IR data by means of the Cotton-Kraihanzel force-field technique.33 This procedure makes certain simplifying assumptions in order to provide a practical solution to a problem that would be extremely difficult to solve rigorously Among the important assumptions are that the C—O vibrations are not coupled to any other vibrational modes of the molecule and that the observed frequencies can be used without correction for enharmonic effects. The results of force constant calculations of this type provide a means of setting up a 77-acceptor series 34... 

The catalytic activity of the zeolitic framework is strongly dependent on the Si Al ratio, i.e. the concentration of the potential catalytic sites. This structural feature, as well as the spectroscopic and energetic properties of the Br0nsted acid sites, has also been investigated by empirical force field techniques. However, in contrast to the adsorption and diffusion phenomena, the stability of the acid sites, and their acid strength is a result of a subtle balance of covalent and ionic bonding interactions, with an active involvement... 

The clear advantage of semi-empirical methods over force field techniques is their... 

As in many other areas of CFD, advances in impeller treatment are occurring rapidly, both in developing new treatments and in making the established treatments more accurate and more efficient computationally. For example, Derksen and Van den Akker (1999) have applied an adaptive force field technique in a model of Rushton turbine flow. VanderHeyden et al. (private communication) at Los Alamos have developed a technique of switching a momentum source term at certain mesh node points on and off in a pattern that simulates the effect of a rotating impeller. These and other approaches promise increased fidelity with greater computational efficiency. 

Merlde (5) argued persuasively that computational chemical force-field techniques are already able to address inportant mechanical nanotechnology problems. Phenomena which are electronic in nature, however, require explicit consideration of the electrons and hence quantum mechanical formulations. Exanples of such phenomena with wide ranging civilian and DoD technological applications include ... 

In open-chain compounds, where rearrangement products are more easily obtained, 2-bromo-2,4-dimethyl-3-pentanone gives the a-alkoxyketone as a major product even when the reaction is carried out under aprotic conditions.Consequently two factors seem to be decisive in inhibiting cyclopropanone formation and yielding the a-alkoxyketone even in an aprotic solvent. The first is the degree of substitution, which can play a role in open-chain or cyclic compounds. The second operates only in cyclic systems and is ring strain which can be calculated by force field techniques. This kind of strain can be due to valence angle deformations or to transannular nonbonded interactions or bonded interactions. ... 

One of the successful examples of employment of QSAR in the design and development of an HIV drug is the discovery of Indinavir (L-735,524) (2), one of the first HIVPI approved by the US-FDA. Holloway et aL [163] first reported this compound when they conducted SAR studies on a combined series of isostere derivatives of (43,44). A high correlation between the inter-molecular interaction energy ( int) calculated for HIVPR inhibitor complexes and enzyme inhibition activity was observed. QSAR 51-53 were developed for native, acetylpepstatin-inhibited and L-689,502-inhibited HIVPR, respectively [ 163]. X-ray coordinates and the force field technique were employed in the calculation of int (intermolecular interaction energy). In these QSAR, rev is the cross-validated correlation coefficient. 

Overall, these articles give a fascinating view of a class of MO techniques that is, on the one hand, capable of providing good quality results for large systems and, on the other hand, can rival the more sophisticated force field techniques for effective use of CPU-time - computational chemistry s Cinderella ... 

Computer experiments particularly use quantum chemical approaches that provide accurate result with intense computational cost. Classical or semiempirical methods on the other hand are able to simulate thousands or up to millions of atoms of a system with pairwise Lennard-Jones (LJ)-type potentials [104-107]. Thus, LJ-type potentials are very accurate for inert gas systems [108], whereas they are unable to describe reactions or they do so by predetermined reactive sites within the molecules of the reactive system [109]. van Duin and coworkers [109-115] developed bond-order-dependent reactive force field technique is called ReaxFF as a solution to the aforementioned problems. Therefore, ReaxFF force field is intended to simulate reactions. They are successfully implemented to study hydrocarbon combustion [112,115,116] that is based on C-H-0 combustion parameters, fuel cell [110,111], metal oxides [117-122], proteins [123,124], phosphates [125,126], and catalyst surface reactions and nanotubes [110-113] based on ReaxFF water parameters [127]. Bond order is the number of chemical bonds between a pair of atoms that depends only on the number and relative positions of other atoms that they interact with [127]. Parameterization of ReaxFFs is achieved using experimental and quantum mechanical data. Therefore, ReaxFF calculations are fairly accurate and robust. The total energy of the molecule is calculated as the combination of bonded and nonbonded interaction energies. 

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