2011-08-25 23:34:50 +08:00
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<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>fix gcmc command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID gcmc N X M type seed T mu displace keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>gcmc = style name of this fix command
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<LI>N = invoke this fix every N steps
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<LI>X = average number of GCMC exchanges to attempt every N steps
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<LI>M = average number of MC moves to attempt every N steps
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<LI>type = atom type to assign to inserted atoms (offset for molecule insertion)
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<LI>seed = random # seed (positive integer)
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<LI>T = temperature of the ideal gas reservoir (temperature units)
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<LI>mu = chemical potential of the ideal gas reservoir (energy units)
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<LI>translate = maximum Monte Carlo translation distance (length units)
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<LI>zero or more keyword/value pairs may be appended to args
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<PRE>keyword = <I>mol</I>, <I>region</I>, <I>maxangle</I>, <I>pressure</I>, <I>fugacity_coeff</I>, <I>full_energy</I>, <I>charge</I>, <I>group</I>, <I>grouptype</I>, or <I>intra_energy</I>
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<I>mol</I> value = template-ID
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template-ID = ID of molecule template specified in a separate <A HREF = "molecule.html">molecule</A> command
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<I>shake</I> value = fix-ID
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fix-ID = ID of <A HREF = "fix_shake.html">fix shake</A> command
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<I>region</I> value = region-ID
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region-ID = ID of region where MC moves are allowed
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<I>maxangle</I> value = maximum molecular rotation angle (degrees)
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<I>pressure</I> value = pressure of the gas reservoir (pressure units)
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<I>fugacity_coeff</I> value = fugacity coefficient of the gas reservoir (unitless)
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<I>full_energy</I> = compute the entire system energy when performing MC moves
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<I>charge</I> value = charge of inserted atoms (charge units)
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<I>group</I> value = group-ID
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group-ID = group-ID for inserted atoms (string)
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<I>grouptype</I> values = type group-ID
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type = atom type (int)
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group-ID = group-ID for inserted atoms (string)
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<I>intra_energy</I> value = intramolecular energy (energy units)
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2011-08-25 23:34:50 +08:00
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 2 gas gcmc 10 1000 1000 2 29494 298.0 -0.5 0.01
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fix 3 water gcmc 10 100 100 0 3456543 3.0 -2.5 0.1 mol my_one_water maxangle 180 full_energy
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fix 4 my_gas gcmc 1 10 10 1 123456543 300.0 -12.5 1.0 region disk
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2011-08-25 23:34:50 +08:00
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>This fix performs grand canonical Monte Carlo (GCMC) exchanges of
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atoms or molecules of the given type with an imaginary ideal gas reservoir at
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the specified T and chemical potential (mu) as discussed in
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<A HREF = "#Frenkel">(Frenkel)</A>. If used with the <A HREF = "fix_nh.html">fix nvt</A> command,
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simulations in the grand canonical ensemble (muVT, constant chemical
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potential, constant volume, and constant temperature) can be
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performed. Specific uses include computing isotherms in microporous
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materials, or computing vapor-liquid coexistence curves.
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</P>
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<P>Every N timesteps the fix attempts a number of GCMC exchanges (insertions
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or deletions) of gas atoms or molecules of
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the given type between the simulation cell and the imaginary
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reservoir. It also attempts a number of Monte Carlo
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moves (translations and molecule rotations) of gas of the given type
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within the simulation cell or region. The average number of
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attempted GCMC exchanges is X. The average number of attempted MC moves is M.
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M should typically be chosen to be
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approximately equal to the expected number of gas atoms or molecules
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of the given type within the simulation cell or region,
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which will result in roughly one
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MC translation per atom or molecule per MC cycle.
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</P>
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<P>For MC moves of molecular gasses, rotations and translations are each
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attempted with 50% probability. For MC moves of atomic gasses,
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translations are attempted 100% of the time. For MC exchanges of
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either molecular or atomic gasses, deletions and insertions are each
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attempted with 50% probability.
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2012-11-14 05:46:55 +08:00
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</P>
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2015-05-09 01:28:15 +08:00
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<P>All inserted particles are always assigned to two groups: the default group
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"all" and the group specified in the fix gcmc command (which can also
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be "all"). In addition, particles are also added to any groups specified
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by the <I>group</I> and <I>grouptype</I> keywords.
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If inserted particles are individual atoms, they are
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assigned the specified atom type. If they are molecules, the type of
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each atom in the inserted molecule is specified in the file read by
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the <A HREF = "molecule.html">molecule</A> command, and those values are added to
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the specified atom type. E.g. if <I>type</I> = 2, and the file specifies
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atom types 1,2,3, then the inserted molecule will have atom types
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3,4,5.
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2014-06-28 07:45:33 +08:00
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</P>
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<P>This fix cannot be used to perform MC insertions of gas atoms or
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molecules other than the exchanged type, but MC deletions,
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translations, and rotations can be performed on any atom/molecule in
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the fix group. All atoms in the simulation cell can be moved using
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regular time integration translations, e.g. via
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<A HREF = "fix_nvt.html">fix_nvt</A>, resulting in a hybrid GCMC+MD simulation. A
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smaller-than-usual timestep size may be needed when running such a
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hybrid simulation, especially if the inserted molecules are not well
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equilibrated.
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</P>
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2015-01-24 01:24:00 +08:00
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<P>This command may optionally use the <I>region</I> keyword to define an
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exchange and move volume. The specified region must have been
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previously defined with a <A HREF = "region.html">region</A> command. It must be
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defined with side = <I>in</I>. Insertion attempts occur only within the
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specified region. For non-rectangular regions, random trial
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points are generated within the rectangular bounding box until a point is found
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that lies inside the region. If no valid point is generated after 1000 trials,
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no insertion is performed, but it is counted as an attempted insertion.
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Move and deletion attempt candidates are selected
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from gas atoms or molecules within the region. If there are no candidates,
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no move or deletion is performed, but it is counted as an attempt move
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or deletion. If an attempted move places the atom or molecule center-of-mass outside
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the specified region, a new attempted move is generated. This process is repeated
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until the atom or molecule center-of-mass is inside the specified region.
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2012-08-08 22:16:01 +08:00
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</P>
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2011-08-25 23:34:50 +08:00
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<P>If used with <A HREF = "fix_nvt.html">fix_nvt</A>, the temperature of the imaginary
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reservoir, T, should be set to be equivalent to the target temperature
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used in <A HREF = "fix_nvt.html">fix_nvt</A>. Otherwise, the imaginary reservoir
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will not be in thermal equilibrium with the simulation cell.
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</P>
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<P>Note that neighbor lists are re-built every timestep that this fix is
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invoked, so you should not set N to be too small. However, periodic
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rebuilds are necessary in order to avoid dangerous rebuilds and missed
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interactions. Specifically, avoid performing so many MC translations
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per timestep that atoms can move beyond the neighbor list skin
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distance. See the <A HREF = "neighbor.html">neighbor</A> command for details.
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</P>
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2015-07-08 10:31:04 +08:00
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<P>When an atom or molecule is to be inserted, its
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coordinates are chosen at a random position within the current
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simulation cell or region, and new atom velocities are randomly chosen from
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the specified temperature distribution given by T. Relative
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coordinates for atoms in a molecule are taken from the template
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molecule provided by the user, with the origin of the relative
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coordinates coinciding with the chosen insertion point. This means
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that if the origin of the template molecule coordinate system
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lies far from the center of the template molecule,
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the inserted molecule will lie far from the insertion point.
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A random initial rotation is used in
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the case of molecule insertions.
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</P>
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<P>Individual atoms are inserted, unless the <I>mol</I> keyword is used. It
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specifies a <I>template-ID</I> previously defined using the
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<A HREF = "molecule.html">molecule</A> command, which reads a file that defines the
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molecule. The coordinates, atom types, charges, etc, as well as any
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bond/angle/etc and special neighbor information for the molecule can
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be specified in the molecule file. See the <A HREF = "molecule.html">molecule</A>
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command for details. The only settings required to be in this file
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are the coordinates and types of atoms in the molecule.
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</P>
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<P>When not using the <I>mol</I> keyword, you should ensure you do not delete
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atoms that are bonded to other atoms, or LAMMPS will
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soon generate an error when it tries to find bonded neighbors. LAMMPS will
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warn you if any of the atoms eligible for deletion have a non-zero
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molecule ID, but does not check for this at the time of deletion.
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</P>
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2014-06-28 07:45:33 +08:00
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<P>If you wish to insert molecules via the <I>mol</I> keyword, that will have
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their bonds or angles constrained via SHAKE, use the <I>shake</I> keyword,
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specifying as its value the ID of a separate <A HREF = "fix_shake.html">fix
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shake</A> command which also appears in your input script.
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</P>
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<P>Optionally, users may specify the maximum rotation angle for
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molecular rotations using the <I>maxangle</I> keyword and specifying
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the angle in degrees. Rotations are performed by generating a random
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point on the unit sphere and a random rotation angle on the
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range [0,maxangle). The molecule is then rotated by that angle about an
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axis passing through the molecule center of mass. The axis is parallel
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to the unit vector defined by the point on the unit sphere. The
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same procedure is used for randomly rotating molecules when they
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are inserted, except that the rotation axis passes through whatever
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origin is used for the molecule template, and the maximum angle is
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360 degrees.
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</P>
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<P>Note that fix GCMC does not use configurational bias
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MC or any other kind of sampling of intramolecular degrees of freedom.
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Inserted molecules can have different orientations, but they will all
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have the same intramolecular configuration,
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which was specified in the molecule command input.
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</P>
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<P>For atomic gasses, inserted atoms have the specified atom type, but
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deleted atoms are any atoms that have been inserted or that belong
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to the user-specified fix group. For molecular gasses, exchanged
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molecules use the same atom types as in the template molecule
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supplied by the user. In both cases, exchanged
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atoms/molecules are assigned to two groups: the default group "all"
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and the group specified in the fix gcmc command (which can also be
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"all").
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</P>
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2013-03-28 06:37:51 +08:00
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<P>The gas reservoir pressure can be specified using the <I>pressure</I>
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keyword, in which case the user-specified chemical potential is
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ignored. For non-ideal gas reservoirs, the user may also specify the
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fugacity coefficient using the <I>fugacity_coeff</I> keyword.
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</P>
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<P>The <I>full_energy</I> option means that fix GCMC will compute the total
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potential energy of the entire simulated system. The total system
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energy before and after the proposed GCMC move is then used in the
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Metropolis criterion to determine whether or not to accept the
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proposed GCMC move. By default, this option is off, in which case
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only partial energies are computed to determine the difference in
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energy that would be caused by the proposed GCMC move.
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</P>
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<P>The <I>full_energy</I> option is needed for systems with complicated
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potential energy calculations, including the following:
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</P>
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<UL><LI> long-range electrostatics (kspace)
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<LI> many-body pair styles
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<LI> hybrid pair styles
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<LI> eam pair styles
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<LI> triclinic systems
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<LI> need to include potential energy contributions from other fixes
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</UL>
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<P>In these cases, LAMMPS will automatically apply the <I>full_energy</I>
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keyword and issue a warning message.
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</P>
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2015-05-12 07:15:26 +08:00
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<P>When the <I>mol</I> keyword is used, the <I>full_energy</I> option also includes
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the intramolecular energy of inserted and deleted molecules. If this
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is not desired, the <I>intra_energy</I> keyword can be used to define an
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amount of energy that is subtracted from the final energy when a molecule
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is inserted, and added to the initial energy when a molecule is
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deleted. For molecules that have a non-zero intramolecular energy, this
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will ensure roughly the same behavior whether or not the <I>full_energy</I>
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option is used.
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</P>
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2015-01-24 01:24:00 +08:00
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<P>Some fixes have an associated potential energy. Examples of such fixes
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include: <A HREF = "fix_efield.html">efield</A>, <A HREF = "fix_gravity.html">gravity</A>,
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<A HREF = "fix_addforce.html">addforce</A>, <A HREF = "fix_langevin.html">langevin</A>,
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<A HREF = "fix_restrain.html">restrain</A>, <A HREF = "fix_temp_berendsen.html">temp/berendsen</A>,
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<A HREF = "fix_temp_rescale.html">temp/rescale</A>, and <A HREF = "fix_wall.html">wall fixes</A>.
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For that energy to be included in the total potential energy of the
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system (the quantity used when performing GCMC moves),
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you MUST enable the <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option for
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that fix. The doc pages for individual <A HREF = "fix.html">fix</A> commands
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specify if this should be done.
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</P>
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<P>Use the <I>charge</I> option to insert atoms with a user-specified point
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charge. Note that doing so will cause the system to become non-neutral.
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LAMMPS issues a warning when using long-range electrostatics (kspace)
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with non-neutral systems. See the
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<A HREF = "compute_group_group.html">compute_group_group</A> documentation for more
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details about simulating non-neutral systems with kspace on.
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</P>
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2014-06-28 07:45:33 +08:00
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<P>Use of this fix typically will cause the number of atoms to fluctuate,
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therefore, you will want to use the
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<A HREF = "compute_modify.html">compute_modify</A> command to insure that the
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current number of atoms is used as a normalizing factor each time
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temperature is computed. Here is the necessary command:
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</P>
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<PRE>compute_modify thermo_temp dynamic yes
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</PRE>
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2011-08-25 23:34:50 +08:00
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<P>If LJ units are used, note that a value of 0.18292026 is used by this
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fix as the reduced value for Planck's constant. This value was
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2015-07-08 03:58:36 +08:00
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derived from LJ parameters for argon, where h* = h/sqrt(sigma^2 *
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2011-08-25 23:34:50 +08:00
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epsilon * mass), sigma = 3.429 angstroms, epsilon/k = 121.85 K, and
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mass = 39.948 amu.
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</P>
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2015-05-09 01:28:15 +08:00
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<P>The <I>group</I> keyword assigns all inserted atoms to the <A HREF = "group.html">group</A>
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2015-07-08 03:58:36 +08:00
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of the group-ID value. The <I>grouptype</I> keyword assigns all
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2015-05-09 01:28:15 +08:00
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inserted atoms of the specified type to the <A HREF = "group.html">group</A>
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of the group-ID value.
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</P>
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2011-08-25 23:34:50 +08:00
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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2015-01-24 01:24:00 +08:00
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<P>This fix writes the state of the fix to <A HREF = "restart.html">binary restart
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2011-08-25 23:34:50 +08:00
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files</A>. This includes information about the random
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number generator seed, the next timestep for MC exchanges, etc. See
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the <A HREF = "read_restart.html">read_restart</A> command for info on how to
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re-specify a fix in an input script that reads a restart file, so that
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the operation of the fix continues in an uninterrupted fashion.
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</P>
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<P>None of the <A HREF = "fix_modify.html">fix_modify</A> options are relevant to this
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fix.
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</P>
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2015-01-24 01:24:00 +08:00
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<P>This fix computes a global vector of length 8, which can be accessed
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2011-08-26 01:01:01 +08:00
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by various <A HREF = "Section_howto.html#howto_15">output commands</A>. The vector
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2015-01-24 01:24:00 +08:00
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values are the following global cumulative quantities:
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2011-08-25 23:34:50 +08:00
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</P>
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2015-07-08 03:58:36 +08:00
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<UL><LI>1 = translation attempts
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<LI>2 = translation successes
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2012-11-14 05:46:55 +08:00
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<LI>3 = insertion attempts
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<LI>4 = insertion successes
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<LI>5 = deletion attempts
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<LI>6 = deletion successes
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<LI>7 = rotation attempts
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<LI>8 = rotation successes
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2011-08-25 23:34:50 +08:00
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</UL>
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<P>The vector values calculated by this fix are "extensive".
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
|
2011-08-27 02:53:00 +08:00
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<P>This fix is part of the MC package. It is only enabled if LAMMPS was
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built with that package. See the <A HREF = "Section_start.html#start_3">Making
|
2011-08-25 23:34:50 +08:00
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LAMMPS</A> section for more info.
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</P>
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<P>Do not set "neigh_modify once yes" or else this fix will never be
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called. Reneighboring is required.
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</P>
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2012-11-14 05:46:55 +08:00
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<P>Can be run in parallel, but aspects of the GCMC part will not scale
|
2015-01-24 01:24:00 +08:00
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well in parallel. Only usable for 3D simulations.
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2012-11-14 05:46:55 +08:00
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</P>
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<P>Note that very lengthy simulations involving insertions/deletions of
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billions of gas molecules may run out of atom or molecule IDs and
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trigger an error, so it is better to run multiple shorter-duration
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simulations. Likewise, very large molecules have not been tested
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and may turn out to be problematic.
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</P>
|
2014-06-28 07:45:33 +08:00
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<P>Use of multiple fix gcmc commands in the same input script can be
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|
problematic if using a template molecule. The issue is that the
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user-referenced template molecule in the second fix gcmc command
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|
may no longer exist since it might have been deleted by the first
|
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fix gcmc command. An existing template molecule will need to be
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referenced by the user for each subsequent fix gcmc command.
|
2012-12-11 03:55:25 +08:00
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</P>
|
2011-08-25 23:34:50 +08:00
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<P><B>Related commands:</B>
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</P>
|
2015-01-24 01:24:00 +08:00
|
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<P><A HREF = "fix_atom_swap.html">fix_atom_swap</A>,
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<A HREF = "fix_nvt.html">fix_nvt</A>, <A HREF = "neighbor.html">neighbor</A>,
|
2012-11-14 05:46:55 +08:00
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<A HREF = "fix_deposit.html">fix_deposit</A>, <A HREF = "fix_evaporate.html">fix_evaporate</A>,
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<A HREF = "delete_atoms.html">delete_atoms</A>
|
2011-08-25 23:34:50 +08:00
|
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</P>
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<P><B>Default:</B>
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</P>
|
2015-05-19 00:10:20 +08:00
|
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<P>The option defaults are mol = no, maxangle = 10, full_energy = no,
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|
except for the situations where full_energy is required, as
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listed above.
|
2011-08-25 23:34:50 +08:00
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</P>
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<HR>
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<A NAME = "Frenkel"></A>
|
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<P><B>(Frenkel)</B> Frenkel and Smit, Understanding Molecular Simulation,
|
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|
|
Academic Press, London, 2002.
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</P>
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</HTML>
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