<p>The (xi - center-of-mass) term is computed taking into account
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
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>
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<p>No information about this fix is written to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>. None of the <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">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"><span class="doc">run</span></a> command.
This fix is not invoked during <a class="reference internal" href="minimize.html"><span class="doc">energy minimization</span></a>.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>respa</em> option is supported by this
fix. This allows to set at which level of the <a class="reference internal" href="run_style.html"><span class="doc">r-RESPA</span></a>
integrator the fix is adding its forces. Default is the outermost level.</p>
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