Single point charge model

If separate D-RESP charge sets are fitted for every single one of the 36 frames, the standard deviation of the electrostatic field generated varies between 3.5 and 5% with respect to the full quantum reference. This accuracy is the best (in the least-squares sense) that can be obtained if the system is modeled with time-dependent atomic point-charges and represents the accuracy limit for a fluctuating point charge model of the dipeptide. 

This field is not the physical electrostatic field observed in the crystal because the ionic model takes no account of the distribution of electrons within the atom. The distributed charge of the atom is replaced by a single point charge at its centre. 

The Born ion is the simplest model of solvation It considers the solvation energy of a spherical, nonpolarizable (sp = 1) solute of radius R with a single point charge of magnitude z at its center. In this case, an analytical expression is available for the solvation energy ... 

If the two charges of the dipole featuring in the layer model are replaced with a single point charge, then the one-layer continuous model relating to electrolyte solutions is obtained. According to this model, the part of the free energy of solvation due to electrostatic interactions is [Ab 78 ]... 

Multicomponent systems that present polyamorphism have also been reported in computer simulation studies. For example, in Ref. [35], it is found that silica has a LLCP at very low temperature. Silica is also a tetrahedral liquid and it shares many of the thermodynamic properties observed in water. In Ref. [35], two silica models were considered. In both models, the interactions among O and Si atoms are isotropic, due to single point charges and short-range interacting sites located on each atom. Both models considered in Ref. [35] are characterized by a LLCP at very low temperature and coexistence between two liquids is observed in out of equilibrium simulations close to one of the spinodal lines (see Fig. 2b). The location of the LLCP was estimated to be below the glass transition in real silica and hence, unaccessible in experiments. We note that polyamorphism in the glass state is indeed observed in compression experiments on amorphous silica [14], and is qualitatively reproduced in computer simulations [89]. Other examples of multicomponent systems that show LLPT in simulations are presented in Refs [65,90]. In these cases, a substance that already shows polymorphism is mixed with a second component. 

The single point charge (SPC) and extended single point charge (SPC/E) water models have been used with these parameters. Although the bulk properties of SPC/E are better than those of SPC, it has been suggested in the literature that SPC is the better choice for interface studies. ... 

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