2006-09-22 00:22:34 +08:00
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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 spring/rg 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 spring/rg K RG0
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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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<LI>spring/rg = style name of this fix command
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<LI>K = harmonic force constant (force/distance units)
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<LI>RG0 = target radius of gyration to constrain to (distance units)
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</UL>
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<PRE> if RG0 = NULL, use the current RG as the target value
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 protein spring/rg 5.0 10.0
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fix 2 micelle spring/rg 5.0 NULL
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</PRE>
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<P><B>Description:</B>
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</P>
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<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).
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This fix is useful to encourage a protein or polymer to fold/unfold
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and also when sampling along the radius of gyration as a reaction
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coordinate (i.e. for protein folding).
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</P>
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<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
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the second and third formulas, when the group is at a different RG
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than the target value RG0.
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</P>
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<CENTER><IMG SRC = "Eqs/fix_spring_rg.jpg">
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</CENTER>
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<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
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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.
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</P>
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<P>If RG0 is specified as NULL, then the RG of the group is computed at
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the time the fix is specified, and that value is used as the target.
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</P>
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2007-10-11 06:28:11 +08:00
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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2007-06-26 08:03:39 +08:00
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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2010-04-03 00:51:06 +08:00
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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 HREF = "Section_howto.html#4_15">output
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2007-10-11 06:28:11 +08:00
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commands</A>. No parameter of this fix can be
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used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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2007-06-26 08:03:39 +08:00
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</P>
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2006-09-22 00:22:34 +08:00
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<P><B>Restrictions:</B> none
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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_spring.html">fix spring</A>, <A HREF = "fix_spring_self.html">fix spring/self</A>
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2008-05-16 06:05:51 +08:00
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<A HREF = "fix_drag.html">fix drag</A>, <A HREF = "fix_smd.html">fix smd</A>
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2006-09-22 00:22:34 +08:00
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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