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