# SPC water model

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) 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
| $r_0$ of OH bond = 1.0
| $\theta_0$ of HOH angle = 109.47$^{\circ}$

Note that as originally proposed, the SPC model was run with a 9
Angstrom cutoff for both LJ and Coulomb terms. It can also be used with
long-range electrostatic solvers (e.g. Ewald or PPPM in LAMMPS) without
changing any of the parameters above, although 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

See the [(Berendsen)](howto-Berendsen) reference for more details on
both the SPC and SPC/E models.

Below is the code for a LAMMPS input file and a molecule file
(`spce.mol`) of SPC/E water for use with the [molecule
command](molecule) demonstrating how to set up a small bulk water system
for SPC/E with rigid bonds.

``` LAMMPS
units real
atom_style full
region box block -5 5 -5 5 -5 5
create_box 2 box  bond/types 1 angle/types 1 &
                extra/bond/per/atom 2 extra/angle/per/atom 1 extra/special/per/atom 2

mass 1 15.9994
mass 2 1.008

pair_style lj/cut/coul/cut 10.0
pair_coeff 1 1 0.1553 3.166
pair_coeff 1 2 0.0    1.0
pair_coeff 2 2 0.0    1.0

bond_style zero
bond_coeff 1 1.0

angle_style zero
angle_coeff 1 109.47

molecule water spce.mol
create_atoms 0 random 33 34564 NULL mol water 25367 overlap 1.33

timestep 1.0
fix rigid     all shake 0.0001 10 10000 b 1 a 1
minimize 0.0 0.0 1000 10000
velocity all create 300.0 5463576
fix integrate all nvt temp 300.0 300.0 100.0

thermo_style custom step temp press etotal density pe ke
thermo 1000
run 20000 upto
write_data spce.data nocoeff




# Water molecule. SPC/E geometry

3 atoms
2 bonds
1 angles

Coords

1    0.00000  -0.06461   0.00000
2    0.81649   0.51275   0.00000
3   -0.81649   0.51275   0.00000

Types

1        1   # O
2        2   # H
3        2   # H

Charges

1       -0.8476
2        0.4238
3        0.4238

Bonds

1   1      1      2
2   1      1      3

Angles

1   1      2      1      3

Shake Flags

1 1
2 1
3 1

Shake Atoms

1 1 2 3
2 1 2 3
3 1 2 3

Shake Bond Types

1 1 1 1
2 1 1 1
3 1 1 1

Special Bond Counts

1 2 0 0
2 1 1 0
3 1 1 0

Special Bonds

1 2 3
2 1 3
3 1 2
```

Wikipedia also has a nice article on [water
models](https://en.wikipedia.org/wiki/Water_model)\_.

------------------------------------------------------------------------

::: {#howto-Berendsen}
**(Berendsen)** Berendsen, Grigera, Straatsma, J Phys Chem, 91,
6269-6271 (1987).
:::