Differences between Force Fields

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

The largest difference between force fields is probably how they handle electrostatics. Each force field uses its own definition of what functions and data should be used. The well-known MM2 force field describes all electrostatic interactions by bond dipoles (4), but most other force fields utilize atomic point charges. The charges may in turn be obtained from fragment matching (34), from bond-type-dependent charge flux (35), or from more complex schemes that can also respond to the environment (36). 

We have seen in the first section how important it is to have an accurate reference geometry for frequency calculations. Therefore, we start with the comparison of empirical and calculated geometries. For the determination of geometries we have used non-local corrections in the exchange correlation potential (LDA/NL). This geometry was used for the determination of LDA force field, and the non-zero forces were taken into account in the calculation of internal force constants and vibrational frequencies. Further, we compare the calculated and observed vibrational frequencies of the transition metal complexes. We also discuss the differences between forc constants of free and complexed small aromatic rings. 

Even for force fields employing the same mathematical form for an energy term there may be significant differences in the parameters Table 2.5 below shows the variability of the parameters for the stretch energy between different force fields. It should be noted that the stretching parameters are among those that vary the least between force fields. 

Transferring torsional and nonbonded terms between force fields is much less reliable. These are lower-energy terms that are much more interdependent. It is quite common to find force fields with significantly different parameters for these eontributions, even when the exact same equations are used. 

Although in teraetion s between vicinal I 4 atom s arc n om in ally treated as non bonded interactions, triost of the force fields treat these somewhat differently from normal 1 5 and greater non-bonded interactions. HyperCbern allows each of these 1 4 non-bonded interactions to be scaled down by a scale factor < 1.0 with AMBHR or OPI-S. bor HIO+ the electrostatic may be scaled and different param eters rn ay be ti sed for I 4 van dcr Waals interactions, fh e. AMBHR force field, for exam pie, n orrn a lly uses a seal in g factor of 0.5 for both van der Waals an d electrostatic interactions. 

Fhe van der Waals and electrostatic interactions between atoms separated by three bonds (i.c. the 1,4 atoms) are often treated differently from other non-bonded interactions. The interaction between such atoms contributes to the rotational barrier about the central bond, in conjunction with the torsional potential. These 1,4 non-bonded interactions are often scaled down by an empirical factor for example, a factor of 2.0 is suggested for both the electrostatic and van der Waals terms in the 1984 AMBER force field (a scale factor of 1/1.2 is used for the electrostatic terms in the 1995 AMBER force field). There are several reasons why one would wish to scale the 1,4 interactions. The error associated wilh the use of an repulsion term (which is too steep compared with the more correct exponential term) would be most significant for 1,4 atoms. In addition, when two 1,4... 

Understanding how the force field was originally parameterized will aid in knowing how to create new parameters consistent with that force field. The original parameterization of a force field is, in essence, a massive curve fit of many parameters from different compounds in order to obtain the lowest standard deviation between computed and experimental results for the entire set of molecules. In some simple cases, this is done by using the average of the values from the experimental results. More often, this is a very complex iterative process. 

In light of the differences between a Morse and a harmonic potential, why do force fields use the harmonic potential First, the harmonic potential is faster to compute and easier to parameterize than the Morse function. The two functions are similar at the potential minimum, so they provide similar values for equilibrium structures. As computer resources expand and as simulations of thermal motion (See Molecular Dynamics , page 69) become more popular, the Morse function may be used more often. 

Another difference between the force fields is the calculation of electrostatic interactions. AMBER, BIO+, and OPLS use point charges to model electrostatic interactions. MM+ calculates electrostatic interactions using bond dipoles. The bond dipole method may not adequately simulate very polar or charged systems. 

---