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Most models
of water usually use three to five interaction sites. SPC (simple point charge)i
and TIP3P (transferrable intermolecular potential with 3 points)ii
are ones out of the most common models that use three interaction-sites (shown
by diagram a in figure 1); the negative charge is located on the oxygen
atom and the positive charge is located and shared on the two hydrogen atoms.

Both SPC and TIP3P models are very similarin SPC the OH bond length is 1.0000 Å
and
the H-O-H bond angle is 109.47°,
and in TIP3P the OH bond length is 0.9572 Å and the H-O-H bond angle is
104.52°. In addition, TIP3P is
commonly used in certain force fields in biological moleculesiii.  Example of four interaction-sites models
(shown by diagram b in figure 1) is TIP4P (transferrable intermolecular potential with 4
points)iv,
the negative charge is not located on the oxygen atom in this four sites model,
instead, it is located on the bisector at a distance of 0.15 Å with a bond length and H-O-H bond angle that are the same
as TIP3P. The most commonly used five sites model
(shown by diagram d in figure 1) is ST2v
which is based on early Ben-Naim and Stillinger model, it has the OH bond
length of 1.0000 Å and the H-O-H bond angle of 109.47°; five sites models are in fact
the only type of model that almost have a tetrahedral shape, whereas three
sites or four sites models are all planar.

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Varies properties of water such as melting temperature, surface tension and phase
diagram can be used to assess the performance of each common models.

 

For melting temperature testvi, the
Gibbs–Duhem technique was used at room pressure for the most
popular models of water SPC, SPC/E, TIP3P and TIP4P,
the result showed that none of the model successfully reproduced the
experimental value for the melting point, giving melting temperatures of -83 °C, -58 °C, -127 °C and -41°C respectively.

When comes to modelling the abnormally high surface tension
of water with the experimental value of 72.75 mJ/m2 at 20 °Cvii,
traditional models TIP3P, TIP4P and TIP5P were considered and compared as well
as newly developed model TIP4P/2005 with results shown in figure 2viii.

The predictions model TIP4P/2005 had almost matching values on surface tension
to the experimental values and thus shows an excellent agreement with real
water.

 

When using water models in reality, the main problems are to
choose the most fitted potential models that would give the best predictions on
certain wanted properties; and the cost of using the chosen models –  as the better models require larger
calculations and therefore require better computational instruments. For
example, one of the most recent model TIP4P-FQ has a very high cost, so as the
cost for further improvement on TIP5Pix;
and relatively cheap models such as TIP3P and SPC have been proven to be
unreliable even in liquid phasex.

Even with well-developed computational calculation methods, the accuracy of
water models can still be questioned because the final decisions on those models
are normative judgements by developers and researchers themselves. Although
water is seen to be a common and simple molecule, it is still very challenging
to model accurately due to all those unexpected characteristics. Since Watts
and Barkerxi
started the development of computer stimulation of water more than 40 years
ago, the best non-polarizable model developed so far is called OPCxii,
and the best polarizable model are iAMOEBAxiii
and BK3xiv;
and it is certain that the improvements on models of water will continue in its
own areas and fields in the future.

i H.

J. C. Berendsen, J. P. M. Postma, W. F. van Gunsteren and J. Hermans, in B.

Pullman (ed.), Intermolecular Forces (Reidel, Dordrecht, 1981)
331.

 

ii W.

L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, and M. L. Klein,
Comparison of simple potential functions for simulating liquid water, Journal
of Chemical Physics, 1983, Volume 79.

 

iii
C. Vega, J. L. F. Abascal, M. M. Conde and J. L. Aragones, What ice can teach
us about water interactions: a critical comparison of the performance of
different water models, Faraday Discussiosn, 2009, volume 141, 251-276.

 

iv W.

L. Jorgensen and J. D. Madura, Temperature and size dependence for monte carlo
simulations of TIP4P water, Molecular Physics, 1985, Volume 56,
1381-1392.

 

v F.

H. Stillinger and A. Rahman, Improved simulation of liquid water by molecular
dynamics, Journal of Chemical Physics, 1974, Volume 60, 1545-1557.

 

vi C.

Vega, E. Sanz, and J. L. F. Abascal, The melting
temperature of the most common models of water, The Journal of Chemical
Physics, 2005, Volume 122, 114507.

 

vii M.

P. Verma, Steam tables for pure water as an ActiveX component in Visual Basic
6.0, Computers and Geosciences, 2003, Volume 29, 1155-1163.

 

viii
C. Vega,
Surface tension of the most popular models of water by using the test-area
simulation method, The Journal of Chemical Physics, 2007,
Volume 126, 154707.

 

ix J.

M. Sorenson, G. Hura, R. M. Glaeser and T. Head-Gordon, What can x-ray
scattering tell us about the radial distribution functions of water? Journal
of Chem. Phys. 2000, Volume 113, 9149-9161.

 

x P.

T. Kiss and A. Baranyai, Sources of the deficiencies in the popular SPC/E and
TIP3P models of water, Journal of Chemical Physics, 2011, Volume 134,
054106.

 

xi J.

A. Barker and R. O. Watts, Chem. Phys. Lett. 1969, Volume 3, 144.

 

xii S.

Izadi, R. Anandakrishnan and A. V. Onufriev, Building water models, A different
approach, Journal of Physical Chemistry Lett, 2014, Volume 5,
3863-3871.

 

xiii
M. L. Laury, L.-P. Wang, V. S. Pande, T, Head-Gordon and J. W. Ponder, Revised
parameters for the AMOEBA polarizable atomic multipole water model, Journal
of Physical Chemistry B, (2015)

 

xiv P.

T. Kiss and A. Baranyai, A new polarizable force field for alkali and halide
ions, Journal of Chemical Physics, 2014, Volume 141,
114501

 

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