tip4p   TIP4P  TIP 4-point, recommended
tip4pew TIP4PEW TIP 4-point with Ewald
tip3p   TIP3P  TIP 3-point
tip5p   TIP5P  TIP 5-point (see http://redmine.gromacs.org/issues/1348 for issues)
tip5pe  TIP5P  TIP 5-point improved for Ewald sums
spc     SPC    simple point charge
spce    SPC/E  extended simple point charge
