# TIP5P water model The five-point TIP5P rigid water model extends the [three-point TIP3P model](Howto_tip3p) by adding two additional sites L, usually massless, where the charge associated with the oxygen atom is placed. These sites L are located at a fixed distance away from the oxygen atom, forming a tetrahedral angle that is rotated by 90 degrees from the HOH plane. Those sites thus somewhat approximate lone pairs of the oxygen and consequently improve the water structure to become even more \"tetrahedral\" in comparison to the [four-point TIP4P model](Howto_tip4p). A suitable pair style with cutoff Coulomb would be: - [pair_style lj/cut/coul/cut](pair_lj_cut_coul) or these commands for a long-range model: - [pair_style lj/cut/coul/long](pair_lj_cut_coul) - [pair_style lj/cut/coul/long/soft](pair_fep_soft) - [kspace_style pppm](kspace_style) - [kspace_style pppm/disp](kspace_style) A TIP5P model *must* be run using a [rigid fix](fix_rigid) since there is no other option to keep this kind of structure rigid in LAMMPS. In order to avoid that LAMMPS produces an error due to the massless L sites, those need to be assigned a tiny non-zero mass. The table below lists the force field parameters (in real [units](units)) to for a the TIP5P model with a cutoff [(Mahoney)](Mahoney) and the TIP5P-E model [(Rick)](Rick) for use with a long-range Coulombic solver (e.g. Ewald or PPPM in LAMMPS). > > Parameter TIP5P TIP5P-E > ------------------------------------------------- ------------------ ------------------ > O mass (amu) 15.9994 15.9994 > H mass (amu) 1.008 1.008 > O charge ($e$) 0.0 0.0 > L charge ($e$) -0.241 -0.241 > H charge ($e$) 0.241 0.241 > LJ $\epsilon$ of OO (kcal/mole) 0.1600 0.1780 > LJ $\sigma$ of OO ($\AA$) 3.1200 3.0970 > LJ $\epsilon$ of HH, LL, OH, OL, HL (kcal/mole) 0.0 0.0 > LJ $\sigma$ of HH, LL, OH, OL, HL ($\AA$) 1.0 1.0 > $r_0$ of OH bond ($\AA$) 0.9572 0.9572 > $\theta_0$ of HOH angle 104.52$^{\circ}$ 104.52$^{\circ}$ > OL distance ($\AA$) 0.70 0.70 > $\theta_0$ of LOL angle 109.47$^{\circ}$ 109.47$^{\circ}$ Below is the code for a LAMMPS input file for setting up a simulation of TIP5P water with a molecule file. Because of using [fix rigid/nvt/small](fix_rigid) no bonds need to be defined and thus no extra storage needs to be reserved for them, but we need to switch to atom style full or use [fix property/atom mol](fix_property_atom) so that fix rigid/nvt/small can identify rigid bodies by their molecule ID: ``` LAMMPS units real atom_style charge atom_modify map array region box block -5 5 -5 5 -5 5 create_box 3 box mass 1 15.9994 mass 2 1.008 mass 3 1.0e-100 pair_style lj/cut/coul/cut 8.0 pair_coeff 1 1 0.160 3.12 pair_coeff 2 2 0.0 1.0 pair_coeff 3 3 0.0 1.0 fix mol all property/atom mol molecule water tip5p.mol create_atoms 0 random 33 34564 NULL mol water 25367 overlap 1.33 timestep 0.5 fix integrate all rigid/nvt/small molecule temp 300.0 300.0 100.0 reset_timestep 0 velocity all create 300.0 5463576 thermo_style custom step temp press etotal density pe ke thermo 1000 run 20000 write_data tip5p.data nocoeff # Water molecule. Explicit TIP5P geometry for use with fix rigid 5 atoms Coords 1 0.00000 -0.06556 0.00000 2 0.75695 0.52032 0.00000 3 -0.75695 0.52032 0.00000 4 0.00000 -0.46971 0.57154 5 0.00000 -0.46971 -0.57154 Types 1 1 # O 2 2 # H 3 2 # H 4 3 # L 5 3 # L Charges 1 0.000 2 0.241 3 0.241 4 -0.241 5 -0.241 ``` Wikipedia also has a nice article on [water models](https://en.wikipedia.org/wiki/Water_model)\_. ------------------------------------------------------------------------ ::: {#Mahoney} **(Mahoney)** Mahoney, Jorgensen, J Chem Phys 112, 8910 (2000) ::: ::: {#Rick} **(Rick)** Rick, J Chem Phys 120, 6085 (2004) :::