forked from lijiext/lammps
91 lines
2.9 KiB
Plaintext
91 lines
2.9 KiB
Plaintext
.. index:: min_modify
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min_modify command
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==================
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Syntax
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""""""
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.. parsed-literal::
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min_modify keyword values ...
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* one or more keyword/value pairs may be listed
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.. parsed-literal::
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keyword = *dmax* or *line*
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*dmax* value = max
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max = maximum distance for line search to move (distance units)
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*line* value = *backtrack* or *quadratic* or *forcezero*
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backtrack,quadratic,forcezero = style of linesearch to use
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Examples
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""""""""
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.. parsed-literal::
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min_modify dmax 0.2
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Description
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"""""""""""
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This command sets parameters that affect the energy minimization
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algorithms selected by the :doc:`min_style <min_style>` command. The
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various settings may affect the convergence rate and overall number of
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force evaluations required by a minimization, so users can experiment
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with these parameters to tune their minimizations.
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The *cg* and *sd* minimization styles have an outer iteration and an
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inner iteration which is steps along a one-dimensional line search in
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a particular search direction. The *dmax* parameter is how far any
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atom can move in a single line search in any dimension (x, y, or z).
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For the *quickmin* and *fire* minimization styles, the *dmax* setting
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is how far any atom can move in a single iteration (timestep). Thus a
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value of 0.1 in real :doc:`units <units>` means no atom will move
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further than 0.1 Angstroms in a single outer iteration. This prevents
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highly overlapped atoms from being moved long distances (e.g. through
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another atom) due to large forces.
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The choice of line search algorithm for the *cg* and *sd* minimization
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styles can be selected via the *line* keyword.
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The default *quadratic* line search algorithm starts out using
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the robust backtracking method described below. However, once
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the system gets close to a local
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minimum and the linesearch steps get small, so that the energy
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is approximately quadratic in the step length, it uses the
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estimated location of zero gradient as the linesearch step,
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provided the energy change is downhill.
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This becomes more efficient than backtracking
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for highly-converged relaxations. The *forcezero*
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line search algorithm is similar to *quadratic*.
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It may be more efficient than *quadratic* on some systems.
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The backtracking search is robust and should always find a local energy
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minimum. However, it will "converge" when it can no longer reduce the
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energy of the system. Individual atom forces may still be larger than
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desired at this point, because the energy change is measured as the
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difference of two large values (energy before and energy after) and
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that difference may be smaller than machine epsilon even if atoms
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could move in the gradient direction to reduce forces further.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`min_style <min_style>`, :doc:`minimize <minimize>`
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Default
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"""""""
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The option defaults are dmax = 0.1 and line = quadratic.
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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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