8.4.4. 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 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 (Berendsen) reference for more details on both the SPC and SPC/E models.
Wikipedia also has a nice article on water models.
(Berendsen) Berendsen, Grigera, Straatsma, J Phys Chem, 91, 6269-6271 (1987).