Molecular Simulation Studies on Thermophysical Properties by Gabriele Raabe

Author:Gabriele Raabe
Language: eng
Format: epub
Publisher: Springer Singapore, Singapore

6.2 Polarizable Force Fields

Regarding current class I force fields, the use of fixed partial charges is considered as a major simplification, as it does not take into account that the electronic distribution of a molecule changes in response to its environment, i.e. the surrounding electrostatic field. Therefore, it is generally believed that the next significant development in force fields (class III) is the explicit inclusion of electronic polarization in molecular modeling in order to obtain molecular models that accurately describe both gas phase and bulk properties. Polarization means the redistribution of charges due to an electrostatic field, and there are different ways to explicitly account for polarization in molecular simulation studies. The most common approach to include electronic polarization into a molecular model is by introducing inducible dipoles, whereas higher order multipole moments are rarely used due to their again higher complexity and computational expense. Widely used methods to generate polarizable force fields are the Drude Oscillators, the fluctuating charge model (charge equilibration) or the induced dipole model. These methods will therefore be discussed briefly in the following subsections, however without considering the presence of external electrostatic potentials. An educational review on the explicit introduction of polarizability into force fields is provided by Yu and van Gunsteren [140]. Applications of polarizable force fields with focus on biological systems are, for instance, reviewed by Halgren and Damm [45] or Ponder and Case [91].

Beyond the high computational expense for the molecular simulations, the explicit introduction of polarization into a force field makes its more complex and therefore the optimization of the parameters more expensive. It has also been shown that the performance of the polarizable force field becomes very sensitive to an inappropriate parametrization [88].

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