Polarizable force fields
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Force fields have recently begun to model electrostatic interactions with explicit charge densities composed of Gaussian functions. A Gaussian multipole formalism is presented which is based on previous work done on Hermite Gaussian functions. The treatment for Gaussian multipoles parallels standard derivations of Cartesian point multipoles. The results obtained for Gaussian multipoles are used to develop a new polarization model based on induced Gaussian dipoles. In contrast to the original induced point dipole model, the induced Gaussian dipole model is capable of finite interactions at short distances. Aspects of convergence related to the induced Gaussian dipole model will be explored. Results for polarization work, energy, and force have been derived for the induced Gaussian dipole model, and a discussion of how the model has been implemented into the AMBER molecular dynamics simulation program is provided. In addition, a method of parameterizing polarizabilities is presented. This method is based on probing a molecule with point charges and fitting polarizabilities to electrostatic potential. In contrast to the generic atom type polarizabilities fit to molecular polarizability tensors, probed polarizabilities are significantly more accurate in terms of reproducing molecular polarizability tensors and electrostatic potential, while retaining conformational transferability. Polarizabilities and atomic partial charges are parameterized for the amino acids, and it is shown that including polarization significantly improves the electrostatic description of point charges over multiple conformations. In addition, a polarizable and non-polarizble model for water and ammonia composed of point charges and induced Gaussian dipoles is presented by fitting to liquid phase heats of vaporization and density. Results are also presented for fitting a polarizable and non-polarizable water model only to ab-initio data, and limitations of the point charge model are discussed.