On the performance of molecular polarization methods. I. Water and carbon tetrachloride close to a point charge

Abstract

The three main methods to implement molecular polarization ͑point dipoles, fluctuating charges, and shell model͒ are tested against high level ab initio calculations for a molecule ͑water, carbon tetrachloride͒ close to a point charge ͑at the distance of a lithium or magnesium ion͒. The goal is to check whether an approximation ͑linear polarization͒ strictly valid at large intermolecular distances is sufficiently accurate for liquid state molecular dynamics simulations, where strong polarization effects are to be expected at short separations. The monitored observable is the molecular dipole moment as a function of the charge-molecule distance for selected molecular orientations. Analytic formulas are derived for the components of the molecular polarization tensor, facilitating the optimization of the performance for each polarization method as a function of its underlying parameters. Overall, the methods studied provide a remarkably good representation of the induced dipole, with no divergences appearing even at the shortest distances. For water close to a monovalent point charge the point dipole model, implemented with one or three dipoles, accurately reproduces the water dipole moment at all distances. Deficiencies appear as the molecular polarizability and/or charge increase: basically, the ab initio induced moments grow faster at intermediate distances than the linear increase characteristic of the phenomenological polarization methods, suggesting that nonlinear effects ͑hyperpolarizability͒ cannot be neglected in these cases. Regarding the capabilities of each method, the point dipole method is the one that performs best overall, with the shell model achieving acceptable results in most instances. The fluctuating charge method shows some noticeable limitations for implementations of comparable complexity ͑in terms of the number of sites required͒.

On the performance of molecular polarization methods. II. Water and carbon tetrachloride close to a cation

“…This effect can be rationalized in terms of the higher attraction exerted by the double charge on the molecular electronic cloud ͑as reported in Ref. 47 the dipole moment induced from a double charge is more than double of the one induced by a single charge͒. As a consequence the molecular electronic cloud is more shifted towards the cation and interacts with the ion outer shell more strongly than for the monovalued ion ͑for a given ion-molecule distance͒.…”

“…In Ref. 47 we reparametrized standard models for both methods; the values of site polarizabilities were fitted in order to reproduce ͑i͒ the polarizability tensor of the molecules and ͑ii͒ the distance dependence of the total dipole moment in the presence of a point charge.…”

“…Both effects ͑increase of the ionic and molecular dipoles with decreasing distance͒ can be mimicked by any polarization method ͑except of course the moderate nonlinear increase discussed in Refs. 47 and 53͒, and therefore should not be a cause of concern to phenomenological molecular polarization methods. If the ion-molecule distance is reduced to contact, though, a different mechanism sets in as evident from the ab initio calculations: repulsion between the electronic clouds of the ion and the molecule, which can be understood as a "mechanical" polarization.…”

“…More important might be the fact that all these methods share a common characteristic: they basically are linear methods and as such they can be expected to fail as nonlinear effects become important. In the simple case study of a point-charge-molecule interaction this was demonstrated to occur at intermediate distances: as the molecule approaches the increasing electric field of the charge 47,53 polarization methods consistently underestimate the induced dipole. It will be shown in this paper that as the distance is further reduced ͑to values typical of first solvation shell molecules͒ different nonlinear effects set in due to electronic cloud overlapping.…”

“…In our previous studies 47,53 we investigated how the most popular polarization methods perform for water or carbon tetrachloride near a mono-or bivalent positive point charge. These two molecules were chosen for their almost complementary electrostatic properties.…”