forked from lijiext/lammps
113 lines
3.4 KiB
Plaintext
113 lines
3.4 KiB
Plaintext
"Higher level section"_Howto.html - "LAMMPS WWW Site"_lws - "LAMMPS
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Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Commands_all.html)
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:line
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TIP4P water model :h3
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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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"pair_style lj/cut/tip4p/cut"_pair_lj.html
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or these two commands for a long-range model:
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"pair_style lj/cut/tip4p/long"_pair_lj.html
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"kspace_style pppm/tip4p"_kspace_style.html :ul
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For both models, the bond lengths and bond angles should be held fixed
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using the "fix shake"_fix_shake.html 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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"(Jorgensen)"_#Jorgensen5. Note that the OM distance is specified in
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the "pair_style"_pair_style.html 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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r0 of OH bond = 0.9572
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theta of HOH angle = 104.52
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OM distance = 0.15
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LJ epsilon of O-O = 0.1550
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LJ sigma of O-O = 3.1536
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LJ epsilon, sigma of OH, HH = 0.0
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Coulombic cutoff = 8.5 :all(b),p
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For the TIP4/Ice model (J Chem Phys, 122, 234511 (2005);
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http://dx.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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r0 of OH bond = 0.9572
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theta of HOH angle = 104.52
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OM distance = 0.1577
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LJ epsilon of O-O = 0.21084
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LJ sigma of O-O = 3.1668
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LJ epsilon, sigma of OH, HH = 0.0
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Coulombic cutoff = 8.5 :all(b),p
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For the TIP4P/2005 model (J Chem Phys, 123, 234505 (2005);
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http://dx.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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r0 of OH bond = 0.9572
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theta of HOH angle = 104.52
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OM distance = 0.1546
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LJ epsilon of O-O = 0.1852
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LJ sigma of O-O = 3.1589
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LJ epsilon, sigma of OH, HH = 0.0
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Coulombic cutoff = 8.5 :all(b),p
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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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r0 of OH bond = 0.9572
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theta of HOH angle = 104.52
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OM distance = 0.1250
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LJ epsilon of O-O = 0.16275
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LJ sigma of O-O = 3.16435
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LJ epsilon, sigma of OH, HH = 0.0 :all(b),p
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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 "pair_style
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lj/cut/tip4p/long"_pair_lj.html command.
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Wikipedia also has a nice article on "water
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models"_http://en.wikipedia.org/wiki/Water_model.
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:line
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:link(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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