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TIP4P water model :h3

The four-point TIP4P rigid water model extends the traditional
three-point TIP3P model by adding an additional site, usually
massless, where the charge associated with the oxygen atom is placed.
This site M is located at a fixed distance away from the oxygen along
the bisector of the HOH bond angle.  A bond style of {harmonic} and an
angle style of {harmonic} or {charmm} should also be used.

A TIP4P model is run with LAMMPS using either this command
for a cutoff model:

"pair_style lj/cut/tip4p/cut"_pair_lj.html

or these two commands for a long-range model:

"pair_style lj/cut/tip4p/long"_pair_lj.html
"kspace_style pppm/tip4p"_kspace_style.html :ul

For both models, the bond lengths and bond angles should be held fixed
using the "fix shake"_fix_shake.html command.

These are the additional parameters (in real units) to set for O and H
atoms and the water molecule to run a rigid TIP4P model with a cutoff
"(Jorgensen)"_#Jorgensen5.  Note that the OM distance is specified in
the "pair_style"_pair_style.html command, not as part of the pair
coefficients.

O mass = 15.9994
H mass = 1.008
O charge = -1.040
H charge = 0.520
r0 of OH bond = 0.9572
theta of HOH angle = 104.52
OM distance = 0.15
LJ epsilon of O-O = 0.1550
LJ sigma of O-O = 3.1536
LJ epsilon, sigma of OH, HH = 0.0
Coulombic cutoff = 8.5 :all(b),p

For the TIP4/Ice model (J Chem Phys, 122, 234511 (2005);
http://dx.doi.org/10.1063/1.1931662) these values can be used:

O mass = 15.9994
H mass =  1.008
O charge = -1.1794
H charge =  0.5897
r0 of OH bond = 0.9572
theta of HOH angle = 104.52
OM distance = 0.1577
LJ epsilon of O-O = 0.21084
LJ sigma of O-O = 3.1668
LJ epsilon, sigma of OH, HH = 0.0
Coulombic cutoff = 8.5 :all(b),p

For the TIP4P/2005 model (J Chem Phys, 123, 234505 (2005);
http://dx.doi.org/10.1063/1.2121687), these values can be used:

O mass = 15.9994
H mass =  1.008
O charge = -1.1128
H charge = 0.5564
r0 of OH bond = 0.9572
theta of HOH angle = 104.52
OM distance = 0.1546
LJ epsilon of O-O = 0.1852
LJ sigma of O-O = 3.1589
LJ epsilon, sigma of OH, HH = 0.0
Coulombic cutoff = 8.5 :all(b),p

These are the parameters to use for TIP4P with a long-range Coulombic
solver (e.g. Ewald or PPPM in LAMMPS):

O mass = 15.9994
H mass = 1.008
O charge = -1.0484
H charge = 0.5242
r0 of OH bond = 0.9572
theta of HOH angle = 104.52
OM distance = 0.1250
LJ epsilon of O-O = 0.16275
LJ sigma of O-O = 3.16435
LJ epsilon, sigma of OH, HH = 0.0 :all(b),p

Note that the when using the TIP4P pair style, the neighbor list
cutoff for Coulomb interactions is effectively extended by a distance
2 * (OM distance), to account for the offset distance of the
fictitious charges on O atoms in water molecules.  Thus it is
typically best in an efficiency sense to use a LJ cutoff >= Coulomb
cutoff + 2*(OM distance), to shrink the size of the neighbor list.
This leads to slightly larger cost for the long-range calculation, so
you can test the trade-off for your model.  The OM distance and the LJ
and Coulombic cutoffs are set in the "pair_style
lj/cut/tip4p/long"_pair_lj.html command.

Wikipedia also has a nice article on "water
models"_http://en.wikipedia.org/wiki/Water_model.

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:link(Jorgensen5)
[(Jorgensen)] Jorgensen, Chandrasekhar, Madura, Impey, Klein, J Chem
Phys, 79, 926 (1983).
