Force field methods errors

Finally, force field methods are zero-dimensional . It is not possible to asses the probable error of a given result within the method. The quality of the result can only be judged by comparison with other calculations on similar types of molecules, for which relevant experimental data exist. 

This study uses empirical force-field methods similar to those used by Kotaka and Kakihana (1977), with a focus on octahedral (XT ) molecules and molecule-like complexes. The reader is cautioned that there appear to be typographical errors in the original tabulation. 

The third chapter maintains the theme of determining conformations by describing how to generate initial structures of organic and bioorganic molecules and how to model experimental NMR data. Andrew Torda and Wilfred van Gunsteren also discuss refinement methods, force fields, systematic errors and biases, and the quality of predicted structures. 

Finally, force field methods are zero-dimensional . It is not possible to asses the probable error of a given result within the method. The quality of the result can only be judged by comparison with other calculations on similar types of molecules, for which "relevant experimental data exist.-----------------------------------------------------... 

Table 5.2 shows a performance of ab initio, DFT, and force field methods, as well as EFP, on the S22 dataset [28]. The second-order perturbation theory, MP2, tends to overestimate the dispersion forces, which becomes obvious from significant errors in describing dispersion-dominated complexes. On the other hand, HF and many popular DFT methods do not describe dispersion at all, again resulting in dramatic errors in dispersion-dominated S5 ems. Augmenting the DFT functionals with dispersion corrections like in BLYP-D3 [29] or >B97X-D [30] dramatically improves their performance. Classical force fields are significantly in error... 

Engler, Andose, and Schleyer (18) used force field methods to predict the heats of formation of numerous polycyclic hydrocarbons experimental data on some of these compounds are now available. Table 2 summarizes this data and indicates the magnitude of the error expected in comparing predicted and experimental heats of formation for compounds of the type being considered. The agreement of calculated with experimental AH°fS is only fair ( 3 kcal mole" ) for both force fields, but experimental values often differed by a comparable... 

In what follows we discuss the individual terms of the VFF-and UBFF-expressions (8) and (9). In doing so we only comment on some aspects concerning the analytical form of these terms. We do not critically review numerical values attached to various potential constants by different authors. Such a discussion, it seems to us, can be dispensed with in view of the fact that many of these values have been derived by trial-and-error methods. Instead, a recently developed powerful optimisation procedure for the systematic determination of potential constants will be outlined in Section 2.4. With this method the results of force field calculations are then only dependent on the analytical form of the force field chosen. 

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). 

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