Many-body polarization

QM/MM Interactions with Coupled Many-Body Polarization, Exchange, and Dispersion... 

Chemistry-Based Force Fields for Polyethylene oxide) with Many-Body Polarization Interactions. 

Following this rationale it can be concluded that the ab initio HF level appears to be the most reliable compromise between accuracy and computational effort to study ions in aqueous solution at present. Despite the shortcomings attributed to a single determi-nantal treatment, the accurate treatment of many-body, polarization, and charge transfer effects in the vicinity of the solute species and the capability to study systems containing hundreds of solvent molecules are key features of QM/MM methods aimed at a reliable description of solution phenomena. However, ongoing hard- and software development will enable the application of more accurate QM techniques within the near future. 

Concluding this section, we are confident that the present treatment of the many-body polarization gives transferable and reliable effective polarizabilities and screening factors. 

Attempts to represent the three-body interactions for water in terms of an analytic function fitted to ab initio results date back to the work of dementi and Corongiu [191] and Niesar et al. [67]. These authors used about 200 three-body energies computed at the Hartree-Fock level and fitted them to parametrize a simple polarization model in which induced dipoles were generated on each molecule by the electrostatic field of other molecules. Thus, the induction effects were distorted in order to describe the exchange effects. The three-body potentials obtained in this way and their many-body polarization extensions have been used in simulations of liquid water. We know now that the two-body potentials used in that work were insufficiently accurate for a meaningful evaluation of the role of three-body effects. 

Giese, T. J., and York, D. M. Charge-dependent model for many-body polarization, exchange, and dispersion interactions in hybrid quantum mechanical/molecular mechanical calculations. /. Chem. Phys., 127, doi 10.1063/l.2778428 [2007]. 

H-bond acceptor with respect to the water molecule, while the two benzenes build a polar H-tt bond with each other. As a result, a H-bonding cycle exists also in this trimer. On the other hand, in the second one-water-two-benzene complex WlB2b, the water donates H-bonds to both benzenes and the many-body polarization becomes destructive. 

Rigid non-polarizable models for water attempt to approximate, via a two-body interaction, the many-body polarization effects which are responsible for a substantial contribution to the properties of the condensed phase of highly polar fluids such as water [58], especially on the dielectric constant [59], by having a large effective dipole moment. While a two-body model might work well in approximating quasi-... 

Numerous polarizable water models have recently been developed [75-82]. At least three types of polarizable models have been used for supercritical water PPC [83], a polarizable TIP-type model [84], and a few variations of SPC-type models with either point or smeared electrostatic charges [78,85,86]. With a few exceptions [87-89] the polarizable models are typically built upon successful non-polarizable counterparts, by scaling the Coulombic charges to match the gas-phase dipole moment, and by including either a polarizable point charge or point dipole to account for the many-body polarization contributions. Moreover, sometimes the permanent dipole moment is set larger than the gas-phase value of 1.85D in order to obtain better agreement with experimental data at ambient conditions [78,82]. 

Current research in water potentials tends to focus on incorporating explicit many-body polarization terms in the water-water energy. This avoids the pairwise additive approach, i.e., the effective media approximation inherent in pairwise additive water potentials, and allows for a better parameterization of the true water-water interaction. Two main avenues for treating polarization effects have developed in the last decade an explicit treatment of classical polarization and fluctuating charge models. The effort expended to find suitable water models will slowly pay off in an enhanced awareness of how to improve current molecular force fields for interactions of other types (e.g., between organic solutes, biomolecules, etc.). 

KLEIN - We do indeed use a semi-empirical model for the various interaction potentials. First, we model the ammonia inter molecular potential with an effective pair potential which ignores many body polarization. Models of this type are remarkably successful in explaining the physical properties of polar fluids. Of course, we really should include many body forces, but at this stage we ignore them. The ammonia potential is fitted to the heat of evaporation and the zero-pressure density. The electron-alkali metal (Lithium) potential is represented by the Shaw pseudo potential fitted to the ionization energy. This is the simplest and crudest model possible. We have explored the effect of using (a) Heine-Abarenkov, (b) Ashcroft, and (c) Phillips-Kleinman forms. Our results are not very sensitive to the choice of pseudo potential. (In the case of Cs metal, which I did not discuss, the sensitivity to the potential is crucial). 

KLEIN - You raise an important question. Long-range many body forces are important. Short-range many body forces are also important. Alas, our knowlegde of the latter is very poor. We used the crudest possible model and have neglected both of these effects. Our electron-ammonia solvent potential includes the most important polarization effect namely, that caused by the solvent dipoles on the electron. We have ignored the self-consistent many body polarization of the coupled electron-solvent system. This latter effect has been discussed by Wallqvist, Thirumalai and Berne (J. Chem. Phys., 1987). I refer you to this article for details. 

O. Borodin and G. D. Smith,/. Fhys. Chem. B, 107(28), 6801-6812 (2003). Development of Quantum Chemistry-Based Force Fields for Poly(Ethylene Oxide) with Many-Body Polarization Interactions. 

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