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SPC water model :h3

The SPC water model specifies a 3-site rigid water molecule with
charges and Lennard-Jones parameters assigned to each of the 3 atoms.
In LAMMPS the "fix shake"_fix_shake.html command can be used to hold
the two O-H bonds and the H-O-H angle rigid.  A bond style of
{harmonic} and an angle style of {harmonic} or {charmm} should also be
used.

These are the additional parameters (in real units) to set for O and H
atoms and the water molecule to run a rigid SPC model.

O mass = 15.9994
H mass = 1.008
O charge = -0.820
H charge = 0.410
LJ epsilon of OO = 0.1553
LJ sigma of OO = 3.166
LJ epsilon, sigma of OH, HH = 0.0
r0 of OH bond = 1.0
theta of HOH angle = 109.47 :all(b),p

Note that as originally proposed, the SPC model was run with a 9
Angstrom cutoff for both LJ and Coulombic terms.  It can also be used
with long-range Coulombics (Ewald or PPPM in LAMMPS), without changing
any of the parameters above, though it becomes a different model in
that mode of usage.

The SPC/E (extended) water model is the same, except
the partial charge assignments change:

O charge = -0.8476
H charge = 0.4238 :all(b),p

See the "(Berendsen)"_#howto-Berendsen reference for more details on both
the SPC and SPC/E models.

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

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:link(howto-Berendsen)
[(Berendsen)] Berendsen, Grigera, Straatsma, J Phys Chem, 91,
6269-6271 (1987).
