forked from lijiext/lammps
179 lines
6.9 KiB
Plaintext
179 lines
6.9 KiB
Plaintext
.. index:: fix msst
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fix msst command
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================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID msst dir shockvel keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* msst = style name of this fix
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* dir = *x* or *y* or *z*
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* shockvel = shock velocity (strictly positive, distance/time units)
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* zero or more keyword value pairs may be appended
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* keyword = *q* or *mu* or *p0* or *v0* or *e0* or *tscale*
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.. parsed-literal::
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*q* value = cell mass-like parameter (mass^2/distance^4 units)
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*mu* value = artificial viscosity (mass/length/time units)
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*p0* value = initial pressure in the shock equations (pressure units)
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*v0* value = initial simulation cell volume in the shock equations (distance^3 units)
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*e0* value = initial total energy (energy units)
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*tscale* value = reduction in initial temperature (unitless fraction between 0.0 and 1.0)
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Examples
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""""""""
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.. parsed-literal::
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fix 1 all msst y 100.0 q 1.0e5 mu 1.0e5
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fix 2 all msst z 50.0 q 1.0e4 mu 1.0e4 v0 4.3419e+03 p0 3.7797e+03 e0 -9.72360e+02 tscale 0.01
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Description
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"""""""""""
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This command performs the Multi-Scale Shock Technique (MSST)
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integration to update positions and velocities each timestep to mimic
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a compressive shock wave passing over the system. See :ref:`(Reed) <Reed>`
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for a detailed description of this method. The MSST varies the cell
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volume and temperature in such a way as to restrain the system to the
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shock Hugoniot and the Rayleigh line. These restraints correspond to
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the macroscopic conservation laws dictated by a shock
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front. *shockvel* determines the steady shock velocity that will be
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simulated.
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To perform a simulation, choose a value of *q* that provides volume
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compression on the timescale of 100 fs to 1 ps. If the volume is not
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compressing, either the shock speed is chosen to be below the material
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sound speed or *p0* has been chosen inaccurately. Volume compression
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at the start can be sped up by using a non-zero value of *tscale*\ . Use
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the smallest value of *tscale* that results in compression.
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Under some special high-symmetry conditions, the pressure (volume)
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and/or temperature of the system may oscillate for many cycles even
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with an appropriate choice of mass-like parameter *q*\ . Such
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oscillations have physical significance in some cases. The optional
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*mu* keyword adds an artificial viscosity that helps break the system
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symmetry to equilibrate to the shock Hugoniot and Rayleigh line more
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rapidly in such cases.
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*tscale* is a factor between 0 and 1 that determines what fraction of
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thermal kinetic energy is converted to compressive strain kinetic
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energy at the start of the simulation. Setting this parameter to a
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non-zero value may assist in compression at the start of simulations
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where it is slow to occur.
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If keywords *e0*\ , *p0*\ ,or *v0* are not supplied, these quantities will
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be calculated on the first step, after the energy specified by
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*tscale* is removed. The value of *e0* is not used in the dynamical
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equations, but is used in calculating the deviation from the Hugoniot.
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Values of shockvel less than a critical value determined by the
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material response will not have compressive solutions. This will be
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reflected in lack of significant change of the volume in the MSST.
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For all pressure styles, the simulation box stays orthogonal in shape.
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Parrinello-Rahman boundary conditions (tilted box) are supported by
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LAMMPS, but are not implemented for MSST.
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This fix computes 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 these commands had been issued:
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.. parsed-literal::
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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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See the :doc:`compute temp <compute_temp>` and :doc:`compute pressure <compute_pressure>` 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". The group for the new computes is "all".
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Restart, fix_modify, output, run start/stop, minimize info
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""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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This fix writes the state of all internal variables to :doc:`binary restart files <restart>`. See the :doc:`read_restart <read_restart>` command
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for info on how to re-specify a fix in an input script that reads a
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restart file, so that the operation of the fix continues in an
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uninterrupted fashion.
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The progress of the MSST can be monitored by printing the global
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scalar and global vector quantities computed by the fix.
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The scalar is the cumulative energy change due to the fix. This is
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also the energy added to the potential energy by the
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:doc:`fix_modify <fix_modify>` *energy* command. With this command, the
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thermo keyword *etotal* prints the conserved quantity of the MSST
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dynamic equations. This can be used to test if the MD timestep is
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sufficiently small for accurate integration of the dynamic
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equations. See also :doc:`thermo_style <thermo_style>` command.
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The global vector contains four values in this order:
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[\ *dhugoniot*\ , *drayleigh*\ , *lagrangian_speed*\ , *lagrangian_position*\ ]
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1. *dhugoniot* is the departure from the Hugoniot (temperature units).
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2. *drayleigh* is the departure from the Rayleigh line (pressure units).
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3. *lagrangian_speed* is the laboratory-frame Lagrangian speed (particle velocity) of the computational cell (velocity units).
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4. *lagrangian_position* is the computational cell position in the reference frame moving at the shock speed. This is usually a good estimate of distance of the computational cell behind the shock front.
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To print these quantities to the log file with descriptive column
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headers, the following LAMMPS commands are suggested:
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.. parsed-literal::
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fix msst all msst z
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fix_modify msst energy yes
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variable dhug equal f_msst[1]
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variable dray equal f_msst[2]
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variable lgr_vel equal f_msst[3]
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variable lgr_pos equal f_msst[4]
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thermo_style custom step temp ke pe lz pzz etotal v_dhug v_dray v_lgr_vel v_lgr_pos f_msst
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These fixes compute a global scalar and a global vector of 4
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quantities, which can be accessed by various :ref:`output commands <howto_15>`. The scalar values calculated
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by this fix are "extensive"; the vector values are "intensive".
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Restrictions
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""""""""""""
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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 :ref:`Making LAMMPS <start_3>` section for more info.
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All cell dimensions must be periodic. This fix can not be used with a
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triclinic cell. The MSST fix has been tested only for the group-ID
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all.
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Related commands
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""""""""""""""""
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:doc:`fix nphug <fix_nphug>`, :doc:`fix deform <fix_deform>`
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Default
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"""""""
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The keyword defaults are q = 10, mu = 0, tscale = 0.01. p0, v0, and e0
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are calculated on the first step.
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----------
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.. _Reed:
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**(Reed)** Reed, Fried, and Joannopoulos, Phys. Rev. Lett., 90, 235503 (2003).
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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