TIP4P-FQ model

The polarizable fluctuating charge model in CHARMM results from the work of Patel, Brooks and co-workers [92, 214], The water model is based on the TIP4P-FQ model of Rick, Stuart and Berne [17], In the development of the force field the electronegativities and hardnesses were treated as empirical parameters and do not have any association with experimental or QM values, for example, from ionization energies and electron affinities of single atoms. 

P ur correlation functions give detailed information about the structure of the liquid. The TIP4P-FQ model gives pair correlation functions that are in good agreement with the neutron diffraction results of Soper and Phillips[23]. For details see Ref. [1]. The static dielectric constant, Co, is calculated from the fluctuations in M, the tot2il dipole of the central simulation box, by... 

Figure 1 Real(top) and imaginary (bottom) parts of the frequency dependent dielectric constant for the TIP4P-FQ model (solid lines), compared to experiment (dotted lines).

Figure 4 Negative charge near the oxygen atom versus time for the TIP4P-FQ water model, comparing the exact (solid line) and extended Lagrangian value (dashed line).

Stuart and Berne [62] have compared the effective two-body TIP4P and a polarizable version based on a fluctuating charge model TIP4P-FQ to solvate either a non-polarizable or a polarizable chloride ion in clusters (H20)nCl for n up to 255. The polarizability model of the ion is based on a Drude oscillator representation. Their simiflations have shown that the chloride ion is solvated at the surface of the cluster for the polarizable model, but the ion is solvated inside the cluster for n < 18 for the non-polarizable model. The explanation for this behaviour is mainly the stronger dipole moment that can be created on water molecules with the polarizable model. Chloride anion polarizability has not been found to have an important effect on the structure of the clusters. 

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