Force field validation

Force field validation. In addition to ensuring that the force field reproduces results of QC calculations we have compared predictions of MD simulations using this force field with the available experimental data. Gas phase MD simulations using the quantum-chemistry based force field accurately reproduced the gas phase structure of DMNA as determined from electron diffraction studies. Liquid phase MD simulations of DMNA predicted the densities and solubility parameter as well as the activation energy and correlation times associated with molecular reorientation that are in good agreement with experimental data [34], As we will show in Section 4, comparison to structural and thermal data for the three pure crystalline polymorphs of HMX support the overall validity of our formulation and parameterization. 

As Table 1 indicates, only a handful of such studies have been reported. In general, the developer of a force field validates the force field by testing and describing its ability to reproduce a selected set of experimental data, including conformational energies comparisons with other force fields, however, are scarce. Moreover, if such comparisons are made, the data set used often is limited, and the overlap of data between different validations is small. In the... 

Ab initio quantum mechanics (QM) can be used to calculate a wide range of molecular and structural properties and, additionally, to derive properties that depend on the electronic structure. Ab initio data from QM calculations can be employed in different stages of force field parameterization. They can be adopted directly into the force field, taken as target data or as initial values in optimization procedures, and may also be used for force field validation. 

Model-based experimental design, long time scale dynamics, virtual measurements, force field validation and robustness, quantum dynamics, dispersion,... 

In solution, complex carbohydrates do not exhibit distinct secondary structural motifs, and any force field must be able to describe the structure of 9 cariwhydrate in terms of an ensemble of conformations. Unfortunately, without extensive experimental data it is often difficult to assess the validity of such a conformational ensemble and force field validation remains a challenge. 

Schmid, F.F.-F. and Meuwly, M. (2008) Direct comparison of experimental and calculated NMR-scalar coupling constants for force field validation and adaptation. J. Chem. Theory Comput., 4, 1949-1958. 

This situation, despite the fact that reliability is increasing, is very undesirable. A considerable effort will be needed to revise the shape of the potential functions such that transferability is greatly enhanced and the number of atom types can be reduced. After all, there is only one type of carbon it has mass 12 and charge 6 and that is all that matters. What is obviously most needed is to incorporate essential many-body interactions in a proper way. In all present non-polarisable force fields many-body interactions are incorporated in an average way into pair-additive terms. In general, errors in one term are compensated by parameter adjustments in other terms, and the resulting force field is only valid for a limited range of environments. 

Validation of a force field is typically done by showing how accurately it reproduces reference data, which may or may not have been used in the actual parameterization. Since different force fields employ different sets of reference data, it is difficult to compare their accuracy directly. Indeed there is no single best force field, each has its advantages and disadvantages. They perform best for the type of compounds used in the parameterization, but may give questionable results for other systems. Table 2.6 gives some typical accuracies for AH( that can be obtained with the MM2 force field. 

The relationship (equation (5.81)) between M and L depends only on fundamental constants, the electronic mass and charge, and does not depend on any of the variables used in the derivation. Although this equation was obtained by applying classical theory to a circular orbit, it is more generally valid. It applies to elliptical orbits as well as to classical motion with attractive forces other than dependence. For any orbit in any central force field, the angular... 

The reliability of molecular mechanics calculations hinges entirely on the validity and range of applicability of the force field. The parameterisation of these functions (the force field) represents the chemistry of the species involved. Many force fields have been developed and the one used in any application usually depends on the molecular mechanics package being used. The force field itself can be validated against experimental and ab initio results. Because of the relative speed of molecular mechanics calculations, it is possible to consider routine calculations of a large number of atoms, certainly tens of thousands, which makes the method amenable to calculations on polymers. To remove surface effects, calculations of bulk properties are normally carried out employing 3D periodic boundaries. In this way it is possible to perform calculations on both amorphous and crystalline systems. 

Having established the validity of the force field for such a wide range of properties the authors go on to extend and apply the force field to electrolyte systems [141]. In particular the diffusion of Li+ in the polymer medium is treated, which is one of the key properties for understanding ionic conduction in the... 

A comparison of force fields as derived and used by different authors is therefore possible only to a limited extent. Any meaningful comparison has to be performed by calculating a large set of molecular properties which comprises the ranges of validity of all force fields under comparison. By experience, calculated structural parameters are generally less sensitive to changes of potential constants than energetical and vibrational quantities. 

Entropies and heat capacities can thus now be calculated using more elaborate models for the vibrational densities of states than the Einstein and Debye models discussed in Chapter 8. We emphasize that the results are only valid in the quasiharmonic approximation and can only be as good as the accuracy of the underlying force-field calculation of such properties can thus be a very sensitive test of interatomic potentials. 

If we are aiming for a quantitatively correct prediction of the behavior and properties of a specific polymer, we need to employ an optimized and carefully validated force field for this specific polymer. In the literature one often finds simulation work using force fields that do not fulfill these criteria but where instead the authors use polymer-xy-like models. Although these models fail to reproduce the properties of the polymer they claim to model... 

Most of the data for these parameterization studies came from ah initio calculations although other sources were also used, in particular, to validate the resulting force field. Thus a set of small model molecules with different conformations of the R—N—C—N—R moiety was calculated at various levels of theory and the results used to derive torsional parameters, hydrogen bond parameters and conformationally dependent correction terms for natural bond lengths and angles, as described below ... 

In this section we briefly outline the explicit form of several general purpose force fields and present the results of calculations on amino and nitro compounds performed as part of their validation. 

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