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update TIP4P howto

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Axel Kohlmeyer
2022-10-27 07:02:16 -04:00
parent 850a303f34
commit e6b67683aa
1 changed files with 24 additions and 13 deletions
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doc/src/Howto_tip4p.rst
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@ -8,18 +8,28 @@ 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 this command
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 two commands for a long-range model:
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>`
For both models, the bond lengths and bond angles should be held fixed
using the :doc:`fix shake <fix_shake>` command.
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
@ -87,15 +97,16 @@ solver (e.g. Ewald or PPPM in LAMMPS):
| 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.
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>`_.