Water multipole fields

Tapia, O., Colonna, F. and Angyan, J. G. Generalized self-consistent reaction field theory in multicenter-multipole ab initio LCGO framework. I. Electronic properties of the water molecule in a Monte Carlo sample of liquid water molecules studied with standard basis sets, J.ChimPhys., (1990), 875-903... 

The second component to the electrostatic interaction is the polarization of the charge distribution by an external field from a permanent multipole. This interaction is always attractive, but it is not pairwise additive and it increases with the number of electrons. For example is the polarizability for water 1.5 A3, while it is 10.6 A3 for benzene. 

The convergence of the solvation energy in the MPE method has been discussed in previous papers from other points of view. Chalmet and Ruiz-Lopez [69] focused on the convergence of the reaction field for the water molecule in liquid water. Curutchet et al. [77] showed that the error in solvation energy for a series of 18 polar molecules in cyclohexane was smaller than 3% when the multicentric multipole moment development was carried out at Zmax = 6. The system considered were CH3F, CHCI3,... 

The quality of the force fields is assessed by comparing the molecular electrostatic potentials, in the vicinity of the molecule, based on the force field and quantum chemical reference calculations. The electrostatic potential is the most suitable observable for such a comparison since it enters directly in the PE potential operator. To probe the electrostatic potential, a grid was constructed around the water molecule, see Ref. [6] for details concerning the construction of this grid. The electrostatic potential, due to the multipoles, at the flth grid point was calculated according to... 

O. Tapia, F. Colonna, and J. G. Angyan, ]. Chim. Phys., 87, 875 (1990). Generalized Self-Consistent Reaction Field Theory in a Multicenter-Multipole Ab Initio LCGO Framework. I. Electronic Properties of the Water Molecule in a Monte Carlo Sample of Liquid Water Molecules Studied with Standard Basis Sets. 

There have been many studies that contrast the accuracy of various atomic charge and distributed multipole models. These studies include the extensive tests provided when various distributed multipole methods were first proposed. For example, there are published contour plots of the potential around a water molecule, the amino acid histidine, and variations in the electrostatic energies of nucleic acid bases,which confirm the significance of the atomic anisotropy shown in the color three-dimensional displays of the electrostatic field around uracil and pyrimidine. It is clear that the difference... 

The COSMO method is also interesting as the basis of a very successful COSMO-RS method, which extends the treatment to solvents other than water [27,28]. The COSMO method is very popular in quantum chemical computations of solvation effects. For example, 29 papers using COSMO calculations were published in 2001. However, we are not aware of its use together with MM force fields. Compared with the BE method, COSMO introduces one more simplification, that of Eq. (22). On the other hand, the matrix A in Eq. (21) is positively defined [25], which makes solution of the system of linear equations simpler and faster. Also, because both A and B matrices contain only electrostatic potential terms, their computation in quantum chemistry is easier than calculation of the electric field terms in Eq. (12). Another potential benefit is that the long-range electrostatic potential contribution is easier to expand into multipoles than the electric field needed in BE methods, which may benefit linear-scaling approaches. 

The theoretical framework developed above is valid in the electric dipole approximation. In this context, it is assumed that the nonlinear polarization PfL(2 >) is reduced to the electric dipole contribution as given in Eq. (1). This assumption is only valid if the surface susceptibility tensor x (2 > >, a>) is large enough to dwarf the contribution from higher orders of the multipole expansion like the electric quadrupole contribution and is therefore the simplest approximation for the nonlinear polarization. At pure solvent interfaces, this may not be the case, since the nonlinear optical activity of solvent molecules like water, 1,2-dichloroethane (DCE), alcohols, or alkanes is rather low. The magnitude of the molecular hyperpolarizability of water, measured by DC electric field induced second harmonic... 

It is has been known that the atomic multipole moments for atoms in AMOEBA model can be calculated through quantum mechanics method and Stone s distributed multipole analysis [61]. Thus, it is straightforward to obtain the parameters of electric multipole potentials based on the distributed multipole analysis after the EMP sites of Gay-Berne particles are decided or directly from AMOEBA force field. However, the EMP parameters of Gay-Berne particles need to be optimized because they are derived based on the gas-phase ab initio quantum mechanics. One possible solution would be to match GBEMP and AMOEBA results for the electrostatic energies between CG particles and water molecules, or between CG particle dimers, at various separations and/or in different orientations. 

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