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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@1630 f3b2605a-c512-4ea7-a41b-209d697bcdaa

This commit is contained in:
sjplimp 2008-03-18 22:56:54 +00:00
parent 1c5ab6d457
commit be95828466
2 changed files with 40 additions and 6 deletions
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21
doc/fix_press_berendsen.html
View File

@ -35,9 +35,10 @@
</PRE>
<LI>zero or more keyword/value pairs may be appended
<LI>keyword = <I>dilate</I>
<LI>keyword = <I>dilate</I> or <I>modulus</I>
<PRE> <I>dilate</I> value = <I>all</I> or <I>partial</I>
<I>modulus</I> value = bulk modulus of system (pressure units)
</PRE>
</UL>
@ -120,6 +121,21 @@ which pressure is relaxed. For example, a value of 1000.0 means to
relax the pressure in a timespan of (roughly) 1000 time units (tau or
fmsec or psec - see the <A HREF = "units.html">units</A> command).
</P>
<P>IMPORTANT NOTE: The relaxation time is actually also a function of the
bulk modulus of the system (inverse of isothermal compressibility).
The bulk modulus has units of pressure and is the amount of pressure
that would need to be applied (isotropically) to reduce the volume of
the system by a factor of 2. The bulk modulus can be set via the
keyword <I>modulus</I>. The <I>Pdamp</I> parameter is effectively multiplied by
the bulk modulus, so if the pressure is relaxing faster than expected
or desired, increasing the bulk modulus has the same effect as
increasing <I>Pdamp</I>. The converse is also true. LAMMPS does not
attempt to guess a correct value of the bulk modulus; it just uses
10.0 as a default value which is about right for a Lennard-Jones
liquid, but will be way off for other materials and way too small for
solids. Thus you should experiment to find appropriate values of
<I>Pdamp</I> and/or the <I>modulus</I> when using this fix.
</P>
<HR>
<P>This fix computes a temperature and pressure each timestep. To do
@ -183,7 +199,8 @@ npt</A>, <A HREF = "fix_temp_berendsen.html">fix temp/berendsen</A>,
</P>
<P><B>Default:</B>
</P>
<P>The keyword defaults are dilate = all.
<P>The keyword defaults are dilate = all, modulus = 10.0 in units of
pressure for whatever <A HREF = "units.html">units</A> are defined.
</P>
<HR>

23
doc/fix_press_berendsen.txt
View File

@ -28,8 +28,9 @@ p-style = {xyz} or {xy} or {yz} or {xz} or {aniso} :l
Pdamp = pressure damping parameter (time units) :pre
zero or more keyword/value pairs may be appended :l
keyword = {dilate} :l
{dilate} value = {all} or {partial} :pre
keyword = {dilate} or {modulus} :l
{dilate} value = {all} or {partial}
{modulus} value = bulk modulus of system (pressure units) :pre
:ule
[Examples:]
@ -111,6 +112,21 @@ which pressure is relaxed. For example, a value of 1000.0 means to
relax the pressure in a timespan of (roughly) 1000 time units (tau or
fmsec or psec - see the "units"_units.html command).
IMPORTANT NOTE: The relaxation time is actually also a function of the
bulk modulus of the system (inverse of isothermal compressibility).
The bulk modulus has units of pressure and is the amount of pressure
that would need to be applied (isotropically) to reduce the volume of
the system by a factor of 2. The bulk modulus can be set via the
keyword {modulus}. The {Pdamp} parameter is effectively multiplied by
the bulk modulus, so if the pressure is relaxing faster than expected
or desired, increasing the bulk modulus has the same effect as
increasing {Pdamp}. The converse is also true. LAMMPS does not
attempt to guess a correct value of the bulk modulus; it just uses
10.0 as a default value which is about right for a Lennard-Jones
liquid, but will be way off for other materials and way too small for
solids. Thus you should experiment to find appropriate values of
{Pdamp} and/or the {modulus} when using this fix.
:line
This fix computes a temperature and pressure each timestep. To do
@ -173,7 +189,8 @@ npt"_fix_npt.html, "fix temp/berendsen"_fix_temp_berendsen.html,
[Default:]
The keyword defaults are dilate = all.
The keyword defaults are dilate = all, modulus = 10.0 in units of
pressure for whatever "units"_units.html are defined.
:line