\(\renewcommand{\AA}{\text{Å}}\)

# 10.4.7. TIP5P water model

The five-point TIP5P rigid water model extends the three-point TIP3P model 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.

A suitable pair style with cutoff Coulomb would be:

- pair_style lj/cut/coul/cut

or these commands for a long-range model:

- pair_style lj/cut/coul/long

- pair_style lj/cut/coul/long/soft

- kspace_style pppm

- kspace_style pppm/disp

A TIP5P model must be run using a rigid fix 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) to for a the TIP5P model with a cutoff (Mahoney) and the TIP5P-E model (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/small no bonds need to be defined and thus no extra storage needs to be reserved for them, but we need to either switch to atom style full or use fix property/atom mol so that fix rigid/small can identify rigid bodies by their molecule ID. Also a neigh_modify exclude command is added to exclude computing intramolecular non-bonded interactions, since those are removed by the rigid fix anyway. For simplicity and speed the example uses a cutoff Coulomb. Most production simulations require long-range Coulomb instead.

    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
    neigh_modify exclude molecule/intra all

    timestep 0.5
    fix integrate all rigid/small molecule langevin 300.0 300.0 50.0 235664
    reset_timestep 0

    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.

(Mahoney) Mahoney, Jorgensen, J Chem Phys 112, 8910 (2000)

(Rick) Rick, J Chem Phys 120, 6085 (2004)