Three-site charge model

In view of the importance of water in chemistry and biology, there have been many attempts to construct simple yet effective intramolecular potentials for water molecules. Water monomers are traditionally left rigid. The early three-site model for water took positive charges on the hydrogens ( h) and a negative charge (qo = on the oxygen, and wrote the pair potential between two... 

One other aspect of nonprimitive electric double layer theories which is particularly relevant to the inner Stern region are the models for the water molecule and the ions. The simplest models for a water molecule and an ion are a hard-sphere point dipole and point charge, respectively. A more realistic model of the hard-sphere water molecule would include quadrupoles and octupoles and also polarizability. However the hard-sphere property is best avoided and replaced, for example, by a Lennard-Jones potential. An alternative to a multipolar water model are three point charge sites associated with the atoms within the water molecule. 

Glattli A, Daura X, van Gunsteren WF (2003) A novel approach for designing simple point charge models for liquid water with three interaction sites, J Comput Chem, 24 1087-1096... 

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... 

Dove and Elston, this interfadal layer can be described by a triple layer snrface com-plexation model (TLM) as shown in Fig. 4.31. The interface consists of three electrostatically charged regions, each with an associated electric potential and snrface charge these are termed the o, p, and d planes. Hydrogen ions are permitted to coordinate with the nnsatnrated sites of the interface at the innermost o layer. Sodinm is positioned at the P layer or the d layer. The surface silicon-oxygen complex may have a different chemical character depending on the adsorbed species, hi a sodium chloride solution the surface complexes can be represented as sSiOHaCl, sSiOHj, =SiOH, =SiO-Na, and SiO". The concentration of each species depends on pH and salt concentration, and the sum of the fractions of these surface species equals 1 ... 

In this study the molecular orbital calculation program employed, ARCANA, utilized only atomic data and iterated to charge self-consistency (see Calculation Note). The substrates, ACh and ASCh, and the products of hydrolysis, choline, thiocholine, and acetic acid, were represented in their entirety, while the AChE enzymic active site was modeled by representing the histidine residue and the serine residue thought to be at the active site by imidazol and methanol respectively. Molecular orbital calculations have been performed for reactants, three possible intermediate acylation complexes, and the acylation products. Since the deacylation mechanism is identical for both ACh and ASCh and has been reported elsewhere (14), those results will not be repeated here. For eacF of the acylation steps, three cases were calculated (a) general acid catalyzed,... 

If a more complex adsorbate is being studied, (N2, CO2, water, etc.) the orientation dependence should be added to the model. For the linear Nj molecule, e.g., a three-site L-J pair potential is typically used [17]. The three sites correspond to the N atoms and the center of mass. The interaction between the atomic sites is taken to be a L-J 12-6 potential function, and partial charges are placed on the three sites in the molecule to reproduce the known quadrupole moment. 

Bottani and Bakaev [23] proposed a two LJ site and a three-charge model (M = 2 and = 3). One positive charge (0.910e) is at the center of the molecular axis joining the two centers of nitrogen atoms and the two symmetric... 

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