119 lines
4.2 KiB
ReStructuredText
119 lines
4.2 KiB
ReStructuredText
TIP4P water model
|
|
=================
|
|
|
|
The four-point TIP4P rigid water model extends the traditional
|
|
three-point TIP3P model by adding an additional site, usually
|
|
massless, where the charge associated with the oxygen atom is placed.
|
|
This site M is located at a fixed distance away from the oxygen along
|
|
the bisector of the HOH bond angle. A bond style of *harmonic* and an
|
|
angle style of *harmonic* or *charmm* should also be used.
|
|
|
|
A TIP4P model is run with LAMMPS using either these commands
|
|
for a cutoff model:
|
|
|
|
* :doc:`pair_style tip4p/cut <pair_lj_cut_tip4p>`
|
|
* :doc:`pair_style lj/cut/tip4p/cut <pair_lj_cut_tip4p>`
|
|
|
|
or these commands for a long-range model:
|
|
|
|
* :doc:`pair_style tip4p/long <pair_coul>`
|
|
* :doc:`pair_style lj/cut/tip4p/long <pair_lj_cut_tip4p>`
|
|
* :doc:`pair_style lj/long/tip4p/long <pair_lj_long>`
|
|
* :doc:`pair_style tip4p/long/soft <pair_fep_soft>`
|
|
* :doc:`pair_style lj/cut/tip4p/long/soft <pair_fep_soft>`
|
|
* :doc:`kspace_style pppm/tip4p <kspace_style>`
|
|
* :doc:`kspace_style pppm/disp/tip4p <kspace_style>`
|
|
|
|
The bond lengths and bond angles should be held fixed using the
|
|
:doc:`fix shake <fix_shake>` or :doc:`fix rattle <fix_shake>` command,
|
|
unless a parameterization for a flexible TIP4P model is used. The
|
|
parameter sets listed below are all for rigid TIP4P model variants and
|
|
thus the bond and angle force constants are not used and can be set to
|
|
any legal value; only equilibrium length and angle are used.
|
|
|
|
These are the additional parameters (in real units) to set for O and H
|
|
atoms and the water molecule to run a rigid TIP4P model with a cutoff
|
|
:ref:`(Jorgensen) <Jorgensen5>`. Note that the OM distance is specified in
|
|
the :doc:`pair_style <pair_style>` command, not as part of the pair
|
|
coefficients.
|
|
|
|
| O mass = 15.9994
|
|
| H mass = 1.008
|
|
| O charge = -1.040
|
|
| H charge = 0.520
|
|
| :math:`r_0` of OH bond = 0.9572
|
|
| :math:`\theta` of HOH angle = 104.52\ :math:`^{\circ}`
|
|
| OM distance = 0.15
|
|
| LJ :math:`\epsilon` of O-O = 0.1550
|
|
| LJ :math:`\sigma` of O-O = 3.1536
|
|
| LJ :math:`\epsilon`, :math:`\sigma` of OH, HH = 0.0
|
|
| Coulomb cutoff = 8.5
|
|
|
|
|
|
|
For the TIP4/Ice model (J Chem Phys, 122, 234511 (2005);
|
|
https://doi.org/10.1063/1.1931662) these values can be used:
|
|
|
|
| O mass = 15.9994
|
|
| H mass = 1.008
|
|
| O charge = -1.1794
|
|
| H charge = 0.5897
|
|
| :math:`r_0` of OH bond = 0.9572
|
|
| :math:`\theta` of HOH angle = 104.52\ :math:`^{\circ}`
|
|
| OM distance = 0.1577
|
|
| LJ :math:`\epsilon` of O-O = 0.21084
|
|
| LJ :math:`\sigma` of O-O = 3.1668
|
|
| LJ :math:`\epsilon`, :math:`\sigma` of OH, HH = 0.0
|
|
| Coulomb cutoff = 8.5
|
|
|
|
|
|
|
For the TIP4P/2005 model (J Chem Phys, 123, 234505 (2005);
|
|
https://doi.org/10.1063/1.2121687), these values can be used:
|
|
|
|
| O mass = 15.9994
|
|
| H mass = 1.008
|
|
| O charge = -1.1128
|
|
| H charge = 0.5564
|
|
| :math:`r_0` of OH bond = 0.9572
|
|
| :math:`\theta` of HOH angle = 104.52\ :math:`^{\circ}`
|
|
| OM distance = 0.1546
|
|
| LJ :math:`\epsilon` of O-O = 0.1852
|
|
| LJ :math:`\sigma` of O-O = 3.1589
|
|
| LJ :math:`\epsilon`, :math:`\sigma` of OH, HH = 0.0
|
|
| Coulomb cutoff = 8.5
|
|
|
|
|
|
|
These are the parameters to use for TIP4P with a long-range Coulombic
|
|
solver (e.g. Ewald or PPPM in LAMMPS):
|
|
|
|
| O mass = 15.9994
|
|
| H mass = 1.008
|
|
| O charge = -1.0484
|
|
| H charge = 0.5242
|
|
| :math:`r_0` of OH bond = 0.9572
|
|
| :math:`\theta` of HOH angle = 104.52\ :math:`^{\circ}`
|
|
| OM distance = 0.1250
|
|
| LJ :math:`\epsilon` of O-O = 0.16275
|
|
| LJ :math:`\sigma` of O-O = 3.16435
|
|
| LJ :math:`\epsilon`, :math:`\sigma` of OH, HH = 0.0
|
|
|
|
|
|
|
Note that the when using the TIP4P pair style, the neighbor list cutoff
|
|
for Coulomb interactions is effectively extended by a distance 2 \* (OM
|
|
distance), to account for the offset distance of the fictitious charges
|
|
on O atoms in water molecules. Thus it is typically best in an
|
|
efficiency sense to use a LJ cutoff >= Coulomb cutoff + 2\*(OM
|
|
distance), to shrink the size of the neighbor list. This leads to
|
|
slightly larger cost for the long-range calculation, so you can test the
|
|
trade-off for your model. The OM distance and the LJ and Coulombic
|
|
cutoffs are set in the :doc:`pair_style lj/cut/tip4p/long
|
|
<pair_lj_cut_tip4p>` command.
|
|
|
|
Wikipedia also has a nice article on `water models <https://en.wikipedia.org/wiki/Water_model>`_.
|
|
|
|
----------
|
|
|
|
.. _Jorgensen5:
|
|
|
|
**(Jorgensen)** Jorgensen, Chandrasekhar, Madura, Impey, Klein, J Chem
|
|
Phys, 79, 926 (1983).
|