Sources of Force Field Parameters

C. Aleman and M. Orozco,/. Comput.-AidedMol. Design, 6,331 (1992). On the Suitability of Semiempirical Calculations as Sources of Force Field Parameters. 

Most of the force fields described in the literature and of interest for us involve potential constants derived more or less by trial-and-error techniques. Starting values for the constants were taken from various sources vibrational spectra, structural data of strain-free compounds (for reference parameters), microwave spectra (32) (rotational barriers), thermodynamic measurements (rotational barriers (33), nonbonded interactions (1)). As a consequence of the incomplete adjustment of force field parameters by trial-and-error methods, a multitude of force fields has emerged whose virtues and shortcomings are difficult to assess, and which depend on the demands of the various authors. In view of this, we shall not discuss numerical values of potential constants derived by trial-and-error methods but rather describe in some detail a least-squares procedure for the systematic optimisation of potential constants which has been developed by Lifson and Warshel some time ago (7 7). Other authors (34, 35) have used least-squares techniques for the optimisation of the parameters of nonbonded interactions from crystal data. Overend and Scherer had previously applied procedures of this kind for determining optimal force constants from vibrational spectroscopic data (36). 

At the time the CLAP force field was proposed, many of the existing ionic liquid models used to borrow parameters from different, not always compatible, sources. For instance, it was common to see parameterizations of the cation and of the anion using information from different force fields [10,11,13], In the development of the CLAP force-field, in order to respect internal consistency, ab initio calculations were used extensively to provide essential data for the development of an internally consistent force field. This included molecular geometry optimization and the description of electron density using extended basis sets, leading to the evaluation of force field parameters such as torsion energy profiles and electrostatic charges on the interaction centers. 

Table 1 Types and Sources of Target Data Used m the Optimization of Empirical Force Field Parameters...

The molecular structures used in the calculations were optimized using a molecular force field program. The force field parameters were derived from various sources The bond length was taken into accound according to a model of Dewar and Llano (7). The VALBOND method of Root et al. (8) was used for the calculation of bond angels. The dihedral angles were parametrized according to the PIMM method (9). 

Most force fields used in coordination chemistry, in respect of the organic part of the molecules, are based on or are at least similar to the MM2 11 or AMBER 11 parameterization schemes, or mixtures thereof. However, it is of importance to stress again that transferring parameters from one force field to another without appropriate checks is not valid. This is not only a question of the different potential energy functions that may be used, but it is also a consequence of the interrelatedness of the entire set of parameters. Force field parameters imported from any source, whether a well-established force field or experimental data, should only be used as a starting point for further parameter refinement. 

FIGURE 44 Distribution of SCxC=0 motif angle (deviation from planarity between the S—C bond and C=0 bond) for the 130 CSD molecules. The geometry of each molecule was energy minimized in vacuo using different levels of theory and force field parameters. See Figure 4.3 for a description of the different plots. Source Adapted from Lupyan et al. [37]. Reproduced with permission of Springer. 

Figure 5.1 Basic specifications of a general, systematic force field for ionic liquids, fa Internal consistency anions and cations are parameterised with the same force field functional, with special attention given to the parameterisation of atomic partial point charges and the flexibility fdihedral angles of the ions. The transfer of parameters from different sources is also checked for adequacy ffor instance, the parameters for the chloride ion are taken from molten salt data, not from aqueous solution results fb Transferability force field parameters are valid within the same homologous family ffor instance, when modelling ions with different alkyl side-chain lengths - bottom double arrow) and allow the possibility of ion interchange to yield different ionic liquids ftop double arrows), (c) Compatibility molecular residues and moieties are taken directly from well-established force fields like OPTS FBI. Simple rules are...

We finish the discussion of force fields developed for imidazolium-based systems by describing the work of Voth and co-workers who simulated [C2mim][N03] first with traditional fixed-charge models and then with a model that included electronic polarizability. For the fixed-charge system, Del Popolo and Voth used a force field having the AMBER function form, with parameters for the cation and anion taken from existing sources. They... 

A great variety of force fields exist, with respect to functional form, to type, and calibration of parameters and to the data used for calibration. The GROMOS force field is one of the simplest forms of a force field for molecular systems. It can be refined and extended in various ways, but a more complex form does not necessarily imply a better force field. If the standard force field does not contain parameters for particular types of atoms, the user should choo.se parameters that are as consistent as possible with the standard force field. The following three common sources of inaccuracy when choosing additional force field parameters are worth mentioning... 

A second source of inaccuracy when choosing force field parameters is the way the torsional interactions are treated. The energetic profile of a rotation around a bond between atoms j and k will depend on... 

A third source of inaccuracy when choosing force field parameters is the treatment of solute-solvent interactions. If solute and solvent force fields are of different types, application of combination rules such as equation (27) may lead to an imbalance between solute-solute, solute-solvent, and solvent-solvent interactions. 

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