TIP4P

In TIP4P there are three interaction classes: the oxygens interact only with other oxygens, and the two charged sites (hydrogen and lone pairs) interact differently with themselves than with the other.

4   # four sites

class 1  O    0.0         0.0  0.0          orient 0.0 0.0 0.0
class 2  H1   0.756950327 0.0 -0.5858822760 orient 0.0 0.0 0.0
class 2  H2  -0.756950327 0.0 -0.5858822760 orient 0.0 0.0 0.0
class 3  lp   0.0         0.0 -0.15         orient 0.0 0.0 0.0

matrix
  # lj sig eps    rep       att
1 1 lj 1.0 0.25  2510.4d3  2552.24
  # need eps = 1/4 to cancel the 4.0 in the LJ potential
  # rep is kJ angstrom^12 / mol
  # att is kJ angstrom^6 / mol

  # coulomb sig k_c          q_1   q_2
2 2 coulomb 1.0 1389.354848  0.52  0.52
2 3 coulomb 1.0 1389.354848  0.52 -1.04
3 3 coulomb 1.0 1389.354848 -1.04 -1.04
                # k_c is measured in kJ/mol