\(\renewcommand{\AA}{\text{Å}}\)
8.4.6. 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 three atoms. In LAMMPS the 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.
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:
See the (Berendsen2) 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 demonstrating how to set up a small bulk water system for
SPC/E with rigid bonds.
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.
(Berendsen2) Berendsen, Grigera, Straatsma, J Phys Chem, 91, 6269-6271 (1987).