Differences in Force Fields

There are many different force fields in use. They differ in three main aspects  

Force field Types Fsir bend Eqq vdw Ed Across Molecules 

ECEPP fixed fixed fixed 12-6 and charge none proteins 

ENCAD 35 P2 P2 imp. 12-6 charge none proteins, nucleic acids 

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In both potentials in eqs. (2.27) and (2.28) the barrier towards linearity is given 2.4 Differences in Force Fields... 

A few comments on the layout of the book. Definitions or common phrases are marked in italic, these can be found in the index. Underline is used for emphasizing important points. Operators, vectors and matrices are denoted in bold, scalars in normal text. Although I have tried to keep the notation as consistent as possible, different branches in computational chemistry often use different symbols for the same quantity. In order to comply with common usage, I have elected sometimes to switch notation between chapters. The second derivative of the energy, for example, is called the force constant k in force field theory, the corresponding matrix is denoted F when discussing vibrations, and called the Hessian H for optimization purposes. 

Raman intensities of the molecular vibrations as well as of their crystal components have been calculated by means of a bond polarizibility model based on two different intramolecular force fields ([87], the UBFF after Scott et al. [78] and the GVFF after Eysel [83]). Vibrational spectra have also been calculated using velocity autocorrelation functions in MD simulations with respect to the symmetry of intramolecular vibrations [82]. 

In this section we will briefly review some of the main different approaches that have been used up to now in order to evaluate the potential energy of each configuration in a Monte Carlo run. As we have already stated, the force fields that describe intra- and intermolecular interactions are at the heart of such statistical calculations because the free energy differences that we want to evaluate are directly dependent on the changes of those interactions. In fact, the important advances of the last ten years in the power of computer techniques for chemical reactions in the condensed phase, that we have mentioned in the Introduction, have been due, to a great extent, to the continual evolution in force fields, with added complexity and improved performance. 

There are several approaches to be classified in the family of focussed models. The common characteristic of all of them is that the system is divided into two (or even more) parts (or layers) which are described at different levels of accuracy. The target layer (the solute) is generally described at QM level (either ab initio or semiempir-ical) while the rest (the solvent) is approximated using an MM description as that used in force fields or a continuum description or both of them in case of more than two layers. 

In these calculations a slightly different intramolecular force field for D20 has been used (69), designed to exactly match the experimental liquid frequencies, the effect on the computed relaxation time is rather small, yielding an almost equal time, within numerical accuracy. 

In dicarbonyls where the CO gronps are chemically different the force field is nnderdetermined with three force constants ( 1, 2, and ku) and only two absorptions which have the same symmetry (a for C, a for C ). Withont any further information, only eqnation (18) can be invoked, since... 

The most extreme difference between force fields arises in the method by which the hydro-... 

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