fix 1 all box/relax xyz 0.0
+fix 1 all box/relax xyz 0.0 vmax 0.001
fix 2 water box/relax aniso 0.0 0.0 1000.0 dilate partial
Description:
@@ -82,24 +83,57 @@ re-scaled. This can be useful for leaving the coordinates of atoms in
a solid substrate unchanged and controlling the pressure of a
surrounding fluid.
+The vmax keyword can be used to limit the fractional change in the
+volume of the simulation box that can occur in one iteration of the
+minimizer. If the pressure is not settling down during the
+minimization this can be because the volume is fluctuating too much.
+The specfied fraction must be greater than 0.0 and should be << 1.0.
+A value of 0.001 means the volume cannot change by more than 1/10 of a
+percent in one iteration for style xyz. For the other styles it
+means no linear dimension of the simulation box can change by more
+than 1/10 of a percent.
+
+IMPORTANT NOTE: As normally computed, pressure includes a kinetic-
+energy or temperature-dependent component; see the compute
+pressure command. However, atom velocities are
+ignored during a minimization, and the applied pressure(s) specified
+with this command are assumed to only be the virial component of the
+pressure (the non-kinetic portion). Thus if atoms have a non-zero
+temperature and you print the usual thermodynamic pressure, it may not
+appear the system is converging to your specified pressure. The
+solution for this is to either (a) zero the velocities of all atoms
+before performing the minimization, or (b) make sure you are
+monitoring the pressure without its kinetic component. The latter
+can be done by outputting the pressure from the fix this command
+creates (see below) or a pressure fix you define yourself.
+
+IMPORTANT NOTE: Because pressure is often a very sensitive function of
+volume, it can be difficult for the minimizer to equilibrate the
+system the desired pressure with high precision. Some techiniques
+that seem to help are (a) use the "min_modify line quadratic" option
+when minimizing with box relaxtions, and (b) minimize several times in
+succession if need be, to drive the pressure closer to the target
+pressure. Also note that some systems (e.g. liquids) will not sustain
+an anisotropic applied pressure, which means the minimizer will not
+converge.
+
This fix computes a temperature and pressure each timestep. The
temperature is used to compute the kinetic contribution to the
-pressure. Since atom velocities do not change during minimization,
-that contribution will depend on what atom velocities are at the
-beginning of the minimization procedure, and will remain constant. To
-do this, the fix creates its own computes of style "temp" and
-"pressure", as if these commands had been issued:
+pressure, even though this is subsequently ignored by default. To do
+this, the fix creates its own computes of style "temp" and "pressure",
+as if these commands had been issued:
compute fix-ID_temp group-ID temp
-compute fix-ID_press group-ID pressure fix-ID_temp
+compute fix-ID_press group-ID pressure fix-ID_temp virial
See the compute temp and compute
pressure commands for details. Note that the
IDs of the new computes are the fix-ID + underscore + "temp" or fix_ID
+ underscore + "press", and the group for the new computes is the same
-as the fix group.
+as the fix group. Also note that the pressure compute does not
+include a kinetic component.
Note that these are NOT the computes used by thermodynamic output (see
the thermo_style command) with ID = thermo_temp
@@ -111,17 +145,6 @@ custom command using the appropriate compute-ID.
It also means that changing attributes of thermo_temp or
thermo_press will have no effect on this fix.
-Like other fixes that perform thermostatting, this fix can be used
-with compute commands that calculate a temperature
-after removing a "bias" from the atom velocities. E.g. removing the
-center-of-mass velocity from a group of atoms or only calculating
-temperature on the x-component of velocity or only calculating
-temperature for atoms in a geometric region. This is not done by
-default, but only if the fix_modify command is used
-to assign a temperature compute to this fix that includes such a bias
-term. See the doc pages for individual compute
-commands to determine which ones include a bias.
-
Restart, fix_modify, output, run start/stop, minimize info:
@@ -132,11 +155,10 @@ files.
The fix_modify temp and press options are
supported by this fix. You can use them to assign a
compute you have defined to this fix which will be used
-in its temperature and pressure calculation, as described above. If
-you do this, note that the kinetic energy derived from the compute
-temperature should be consistent with the virial term computed using
-all atoms for the pressure. LAMMPS will warn you if you choose to
-compute temperature on a subset of atoms.
+in its temperature and pressure calculation, as described above. Note
+that as described above, if you assign a pressure compute to this fix
+that includes a kinetic energy component it will affect the
+minimization, most likely in an undesirable way.
IMPORTANT NOTE: If both the temp and press keywords are used in a
single thermo_modify command (or in two separate commands), then the
@@ -156,8 +178,8 @@ the run command.
This fix is invoked during energy minimization, but
not for the purpose of adding a contribution to the energy or forces
-being minimized. Instead it alters the box geometry as described
-above.
+being minimized. Instead it alters the simulation box geometry as
+described above.
Restrictions:
@@ -171,6 +193,6 @@ periodic.
Default:
-The keyword default is dilate = all.
+
The keyword defaults are dilate = all and vmax = 0.0001.
