Polarizability charge-dipole interaction model

A charge-dipole interaction model has also been used [27] recently for the calculation of the freqnency-dependent polarizability of silver clnslers. Time variations of the atomic charges are shown to be related to the cnrrents that flow through... 

POL and POLl have the same three-site tetrahedral, SPC-like geometry with a charge on each site. The charge magnitude is adjusted to reproduce properties of the liquid, and the resulting dipole, 2.024 D, is much closer to the value in gas phase than for effective non-polarizable models. Polarizability, described by atomic polarizabilities obtained from the atom-dipole interaction model of Applequist et al [172] is however underestimated. POLl modifies the... 

Other investigations using the charge-dipole model include the study of enhanced polarizability of aromatic molecules placed between two silver clusters [32], polarization of fullerenes [28], fullerene clusters [29], silicon clusters [31], and so on. Polarizability of molecular clusters [24] has also been calculated using the dipole interaction model and it would be interesting to investigate the applicability of simple linear scaling relations of polarizability as a function of the cluster size, as obtained by ab initio calculation [56],... 

Because this method avoids iterative calculations to attain the SCF condition, the extended Lagrangian method is a more efficient way of calculating the dipoles at every time step. However, polarizable point dipole methods are still more computationally intensive than nonpolarizable simulations. Evaluating the dipole-dipole interactions in Eqs. (9-7) and (9-20) is several times more expensive than evaluating the Coulombic interactions between point charges in Eq. (9-1). In addition, the requirement for a shorter integration timestep as compared to an additive model increases the computational cost. 

One important difference between the shell model and polarizable point dipole models is in the former s ability to treat so-called mechanical polarization effects. In this context, mechanical polarization refers to any polarization of the electrostatic charges or dipoles that result from causes other than the electric field of neighboring atoms. In particular, mechanical interactions such as steric overlap with nearby molecules can induce polarization in the shell model, as further described below. These mechanical polarization effects are physically realistic and are quite important in some condensed-phase systems. 

Future directions in the development of polarizable models and simulation algorithms are sure to include the combination of classical or semiempir-ical polarizable models with fully quantum mechanical simulations, and with empirical reactive potentials. The increasingly frequent application of Car-Parrinello ab initio simulations methods " may also influence the development of potential models by providing additional data for the validation of models, perhaps most importantly in terms of the importance of various interactions (e.g., polarizability, charge transfer, partially covalent hydrogen bonds, lone-pair-type interactions). It is also likely that we will see continued work toward better coupling of charge-transfer models (i.e., EE and semiem-pirical models) with purely local models of polarization (polarizable dipole and shell models). 

A much more accurate way to include the induced dipole interactions is to compute them for every configuration and include them in the simulation. Barker [16] showed how to make such calculations using an iterative procedure. For point charge models, together with the assumption that the molecular polarizability can... 

Most of the potential energy surfaces reviewed so far have been based on effective pair potentials. It is assumed that the parameterization is such as to account for nonadditive interactions, but in a nonexplicit way. A simple example is the use of a charge distribution with a dipole moment of 2.ID in the ST2 model. However, it is well known that there are significant non-pairwise additive interactions in liquid water and several attempts have been made to include them explicitly in simulations. Nonadditivity can arise in several ways. We have already discussed induced dipole interactions, which are a consequence of the permanent diple moment and polarizability of the molecules. A second type of nonadditive interaction arises from the deformation of the molecules in a condensed phase. Some contributions from such terms are implicitly included in calculations based on flexible molecule potentials. Other contributions arises from electron correlation, exchange, and similar effects. A good example is the Axilrod-Teller three-body dispersion interaction ... 

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