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< title > fix spring/rg command — LAMMPS documentation< / title >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_intro.html" > 1. Introduction< / a > < / li >
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< li > fix spring/rg command< / li >
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< span id = "index-0" > < / span > < h1 > fix spring/rg command< a class = "headerlink" href = "#fix-spring-rg-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< a class = "headerlink" href = "#syntax" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > fix ID group-ID spring/rg K RG0
< / pre > < / div >
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< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "fix.html" > < em > fix< / em > < / a > command< / li >
< li > spring/rg = style name of this fix command< / li >
< li > K = harmonic force constant (force/distance units)< / li >
< li > RG0 = target radius of gyration to constrain to (distance units)< / li >
< / ul >
< div class = "highlight-python" > < div class = "highlight" > < pre > if RG0 = NULL, use the current RG as the target value
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< h2 > Examples< a class = "headerlink" href = "#examples" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > fix 1 protein spring/rg 5.0 10.0
fix 2 micelle spring/rg 5.0 NULL
< / pre > < / div >
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< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > Apply a harmonic restraining force to atoms in the group to affect
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their central moment about the center of mass (radius of gyration).
This fix is useful to encourage a protein or polymer to fold/unfold
and also when sampling along the radius of gyration as a reaction
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coordinate (i.e. for protein folding).< / p >
< p > The radius of gyration is defined as RG in the first formula. The
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energy of the constraint and associated force on each atom is given by
the second and third formulas, when the group is at a different RG
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than the target value RG0.< / p >
< img alt = "_images/fix_spring_rg.jpg" class = "align-center" src = "_images/fix_spring_rg.jpg" / >
< p > The (xi - center-of-mass) term is computed taking into account
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periodic boundary conditions, m_i is the mass of the atom, and M is
the mass of the entire group. Note that K is thus a force constant
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for the aggregate force on the group of atoms, not a per-atom force.< / p >
< p > If RG0 is specified as NULL, then the RG of the group is computed at
the time the fix is specified, and that value is used as the target.< / p >
< / div >
< div class = "section" id = "restart-fix-modify-output-run-start-stop-minimize-info" >
< h2 > Restart, fix_modify, output, run start/stop, minimize info< a class = "headerlink" href = "#restart-fix-modify-output-run-start-stop-minimize-info" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > No information about this fix is written to < a class = "reference internal" href = "restart.html" > < em > binary restart files< / em > < / a > . None of the < a class = "reference internal" href = "fix_modify.html" > < em > fix_modify< / em > < / a > options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various < a class = "reference internal" href = "Section_howto.html#howto-15" > < span > output commands< / span > < / a > . No parameter of this fix can
be used with the < em > start/stop< / em > keywords of the < a class = "reference internal" href = "run.html" > < em > run< / em > < / a > command.
This fix is not invoked during < a class = "reference internal" href = "minimize.html" > < em > energy minimization< / em > < / a > .< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< a class = "headerlink" href = "#restrictions" title = "Permalink to this headline" > ¶< / a > < / h2 >
< blockquote >
< div > none< / div > < / blockquote >
< / div >
< div class = "section" id = "related-commands" >
< h2 > Related commands< a class = "headerlink" href = "#related-commands" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > < a class = "reference internal" href = "fix_spring.html" > < em > fix spring< / em > < / a > , < a class = "reference internal" href = "fix_spring_self.html" > < em > fix spring/self< / em > < / a >
< a class = "reference internal" href = "fix_drag.html" > < em > fix drag< / em > < / a > , < a class = "reference internal" href = "fix_smd.html" > < em > fix smd< / em > < / a > < / p >
< p > < strong > Default:< / strong > none< / p >
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