mirror of https://github.com/lammps/lammps.git
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220 lines
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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 nphug 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 nphug keyword value ...
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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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<PRE>one or more keyword value pairs may be appended
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keyword = <I>temp</I> or <I>iso</I> or <I>aniso</I> or <I>tri</I> or <I>x</I> or <I>y</I> or <I>z</I> or <I>couple</I> or <I>tchain</I> or <I>pchain</I> or <I>mtk</I> or <I>tloop</I> or <I>ploop</I> or <I>nreset</I> or <I>drag</I> or <I>dilate</I> or <I>scaleyz</I> or <I>scalexz</I> or <I>scalexy</I>
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<I>temp</I> values = Value1 Value2 Tdamp
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Value1, Value2 = Nose-Hoover target temperatures, ignored by Hugoniostat
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Tdamp = temperature damping parameter (time units)
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<I>iso</I> or <I>aniso</I> or <I>tri</I> values = Pstart Pstop Pdamp
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Pstart,Pstop = scalar external pressures, must be equal (pressure units)
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Pdamp = pressure damping parameter (time units)
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<I>x</I> or <I>y</I> or <I>z</I> or <I>xy</I> or <I>yz</I> or <I>xz</I> values = Pstart Pstop Pdamp
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Pstart,Pstop = external stress tensor components, must be equal (pressure units)
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Pdamp = stress damping parameter (time units)
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<I>couple</I> = <I>none</I> or <I>xyz</I> or <I>xy</I> or <I>yz</I> or <I>xz</I>
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<I>tchain</I> value = length of thermostat chain (1 = single thermostat)
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<I>pchain</I> values = length of thermostat chain on barostat (0 = no thermostat)
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<I>mtk</I> value = <I>yes</I> or <I>no</I> = add in MTK adjustment term or not
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<I>tloop</I> value = number of sub-cycles to perform on thermostat
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<I>ploop</I> value = number of sub-cycles to perform on barostat thermostat
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<I>nreset</I> value = reset reference cell every this many timesteps
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<I>drag</I> value = drag factor added to barostat/thermostat (0.0 = no drag)
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<I>dilate</I> value = <I>all</I> or <I>partial</I>
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<I>scaleyz</I> value = <I>yes</I> or <I>no</I> = scale yz with lz
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<I>scalexz</I> value = <I>yes</I> or <I>no</I> = scale xz with lz
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<I>scalexy</I> value = <I>yes</I> or <I>no</I> = scale xy with ly
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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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<P>fix myhug all nphug temp 1.0 1.0 10.0 z 40.0 40.0 70.0
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fix myhug all nphug temp 1.0 1.0 10.0 iso 40.0 40.0 70.0 drag 200.0 tchain 1 pchain 0
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</P>
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<P><B>Description:</B>
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</P>
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<P>This command is a variant of the Nose-Hoover
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<A HREF = "fix_nh.html">fix npt</A> fix style.
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It performs time integration of the Hugoniostat equations
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of motion developed by Ravelo et al. <A HREF = "#Ravelo">(Ravelo)</A>.
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These equations compress the system to a state with average
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axial stress or pressure equal to the specified target value
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and that satisfies the Rankine-Hugoniot (RH)
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jump conditions for steady shocks.
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</P>
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<P>The compression can be performed
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either
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hydrostatically (using keyword iso, aniso, or tri) or uniaxially
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(using keywords x, y, or z). In the hydrostatic case,
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the cell dimensions change dynamically so that the average axial stress
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in all three directions converges towards the specified target value.
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In the uniaxial case, the chosen cell dimension changes dynamically
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so that the average
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axial stress in that direction converges towards the target value. The
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other two cell dimensions are kept fixed (zero lateral strain).
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</P>
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<P>This leads to the following additional restrictions on the keywords:
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</P>
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<P>One and only one of the following keywords should be used: iso, aniso, tri, x, y, z,
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</P>
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<P>The specified initial and final target pressures must be the same.
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</P>
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<P>The keywords xy, xz, yz may not be used.
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</P>
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<P>The only admissible value for the couple keyword is xyz,
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which has the same effect as keyword iso
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</P>
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<UL><LI>The temp keyword must be used in order to specify the time constant for
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<LI>kinetic energy relaxation, but initial and final target temperature values
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<LI>are ignored.
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</UL>
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<P>Essentially, a Hugoniostat simulation is an NPT simulation in which the
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user-specified target temperature is replaced with a time-dependent
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target temperature Tt obtained from the following equation:
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</P>
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<P>Tt - T = (0.5*(P+P0)(V0-V)+E0-E)/(Ndof kB) = Delta
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</P>
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<P>where T and Tt are the instantaneous and target temperatures,
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P and P0 are the instantaneous and reference pressure or axial stress,
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depending on whether hydrostatic or uniaxial compression is being
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performed, V and V0 are the instantaneous and reference volumes,
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E and E0 are the instantaneous and reference internal energy (potential
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plus kinetic), Ndof is the number of degrees of freedom used in the
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definition of temperature, and kB is the Boltzmann constant. Delta is the
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negative deviation of the instantaneous temperature from the target temperature.
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The values of E0, V0, and P0 are the instantaneous values at the start of
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the simulation. These can be overridden using the fix_modify keywords <I>e0</I>,
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<I>v0</I>, and <I>p0</I> described below.
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</P>
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<HR>
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<P>IMPORTANT NOTE: Unlike the <A HREF = "fix_temp_berendsen.html">fix
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temp/berendsen</A> command which performs
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thermostatting but NO time integration, this fix performs
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thermostatting/barostatting AND time integration. Thus you should not
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use any other time integration fix, such as <A HREF = "fix_nve.html">fix nve</A> on
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atoms to which this fix is applied. Likewise, this fix should not be
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used on atoms that have their temperature controlled by another fix
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- e.g. by <A HREF = "fix_nh.html">fix langevin</A> or <A HREF = "fix_temp_rescale.html">fix temp/rescale</A>
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commands.
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</P>
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<HR>
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<P>This fix compute a temperature and pressure each timestep. To do
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this, the fix creates its own computes of style "temp" and "pressure",
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as if one of these two sets of commands had been issued:
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</P>
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<PRE>compute fix-ID_temp group-ID temp
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compute fix-ID_press group-ID pressure fix-ID_temp
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</PRE>
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<PRE>compute fix-ID_temp all temp
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compute fix-ID_press all pressure fix-ID_temp
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</PRE>
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<P>See the <A HREF = "compute_temp.html">compute temp</A> and <A HREF = "compute_pressure.html">compute
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pressure</A> commands for details. Note that the
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IDs of the new computes are the fix-ID + underscore + "temp" or fix_ID
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+ underscore + "press". For fix nvt, the group for the new computes
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is the same as the fix group. For fix nph and fix npt, the group for
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the new computes is "all" since pressure is computed for the entire
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system.
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</P>
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<P>Note that these are NOT the computes used by thermodynamic output (see
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the <A HREF = "thermo_style.html">thermo_style</A> command) with ID = <I>thermo_temp</I>
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and <I>thermo_press</I>. This means you can change the attributes of this
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fix's temperature or pressure via the
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<A HREF = "compute_modify.html">compute_modify</A> command or print this temperature
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or pressure during thermodynamic output via the <A HREF = "thermo_style.html">thermo_style
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custom</A> command using the appropriate compute-ID.
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It also means that changing attributes of <I>thermo_temp</I> or
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<I>thermo_press</I> will have no effect on this fix.
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</P>
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<HR>
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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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<P>This fix writes the values of E0, V0, and P0, as well as the
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state of all the thermostat and barostat
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variables to <A HREF = "restart.html">binary restart files</A>. See the
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<A HREF = "read_restart.html">read_restart</A> command for info on how to re-specify
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a fix in an input script that reads a restart file, so that the
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operation of the fix continues in an uninterrupted fashion.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>e0</I>, <I>v0</I> and <I>p0</I> keywords
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can be used to define the values of these quantities. Note the
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the values for <I>e0</I> and <I>v0</I> are extensive, and so must correspond
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to the total energy and volume of the entire system, not energy and
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volume per atom. If any of these quantities are not specified, then the
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instanteous value in the system at the start of the simulation is used.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>temp</I> and <I>press</I> options are
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supported by these fixes. You can use them to assign a
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<A HREF = "compute.html">compute</A> you have defined to this fix which will be used
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in its thermostatting or barostatting procedure, as described above.
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If you do this, note that the kinetic energy derived from the compute
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temperature should be consistent with the virial term computed using
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all atoms for the pressure. LAMMPS will warn you if you choose to
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compute temperature on a subset of atoms.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by these
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fixes to add the energy change induced by Nose/Hoover thermostatting
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and barostatting to the system's potential energy as part of
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<A HREF = "thermo_style.html">thermodynamic output</A>. Either way, this energy is *not*
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included in the definition of internal energy E when calculating the value
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of Delta in the above equation.
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</P>
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<P>These fixes compute a global scalar and a global vector of quantities,
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which can be accessed by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. The scalar value calculated by
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these fixes is "extensive"; the vector values are "intensive".
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</P>
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<P>The scalar is the cumulative energy change due to the fix.
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</P>
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<P>The vector stores three quantities unique to this fix (Delta, Us, and up),
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followed by all the internal Nose/Hoover thermostat and barostat
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variables defined for <A HREF = "fix_nh.html">fix_style npt</A>. Delta is the deviation
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of the temperature from the target temperature, given by the above equation.
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Us and up are the shock and particle velocity correspponding to a steady
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shock calculated from the Rh conditions. They have units of distance/time.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This fix style is part of the SHOCK package. It is only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>All the usual restrictions for <A HREF = "fix_nh.html">fix_style npt</A> apply,
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plus the additional ones mentioned above.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_msst.html">fix msst</A>, <A HREF = "fix_nh.html">fix npt</A>, <A HREF = "fix_modify.html">fix_modify</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The keyword defaults are the same as those for <A HREF = "fix_nh.html">fix npt</A>
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</P>
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<HR>
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<A NAME = "Ravelo"></A>
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<P><B>(Ravelo)</B> Ravelo, Holian, Germann and Lomdahl, Phys Rev B, 70, 014103 (2004).
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</P>
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</HTML>
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