Force field methods advantages

The molecular mechanics (MM) or force field method is an empirical method based on classical mechanics and adjustable parameters. It has the disadvantage of being limited in its application to certain kinds of compounds for which the required parameters have been determined (experimentally or by theoretical calculations). Its advantage is a considerably shorter computation time in comparison with other procedures having the same purpose. This method has been shown to be very reliable and efficient in determing molecular geometries, energies, and other properties for a wide variety of compounds. 

An additional advantage of the force field method 32) is its power to predict the energy levels of conformations which are not populated and even complete rotational potentials of bonds. Again, statisfying agreement with results from dynamic nmr-measurements for a series of crowed hydrocarbons was found 36). Knowledge of the shape of rotational potentials proved to be helpful for the interpretation of entropy effects in these series of compounds and in their thermolysis reactions. 

Semi-empirical methods share the advantage/disadvantage of force field methods, CTor ystems where much experimental information is already... 

The small system contains those atoms treated using high level QM method, while total system consists of all of atoms, which is calculated by classical force field. The advantage of this form is easy to implement, and no explicit treatment of the QM and MM interface is needed. However, it requires MM parameters for full systems, and the treatment of the electronic interaction between inner and outer level is also problematic. 

In 2001, van Duin (Van Duin et al, 2001) proposed a technique of reaction force field, this method was applied to carbohydrates, the geometry data of compounds from simulation are consistent with the literature well, and the bond parameters agree to the results of quantum chemistry, but the calculation time is much less than the time required for quantum chemical calculations. Subsequently, researchers come to realize the advantages of the reaction force field method compared with quantum chemistry and electrostatic force field methods. Reaction force field parameters that are suitable for other materials have gradually been developed, such as silicon oxide, platinum, and titanium. The reaction force field parameters are theoretically universal and general. When Kim et al (2013) studied the interaction between the titanium oxide and water, sodium ions, chloride ions, methanol, and formic acid and other substances, the interaction parameter of Cl/O/H and Na/O/H is from the hteratures of different systems. 

Much work remains to be done in the development of this approach to explore the advantages and limitations of the method. The method will be extended to force fields that include torsional terms large systems such as biological macromolecules will also be treated. 

Large stepsizes result in a strong reduction of the number of force field evaluations per unit time (see left hand side of Fig. 4). This represents the major advantage of the adaptive schemes in comparison to structure conserving methods. On the right hand side of Fig. 4 we see the number of FFTs (i.e., matrix-vector multiplication) per unit time. As expected, we observe that the Chebyshev iteration requires about double as much FFTs than the Krylov techniques. This is due to the fact that only about half of the eigenstates of the Hamiltonian are essentially occupied during the process. This effect occurs even more drastically in cases with less states occupied. 

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