Save the vector array <tt>Rc[j][i]</tt> into file specified by <tt>finalcnfile</tt>.
<tt>Rc</tt> contains the real coordinates of the atoms specified by <tt>constrainatoms</tt>.

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Note that the file format created by <tt>writeRchain is different from that of the <tt>.cn</tt> file.

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Also see <tt>readRchain</tt>, <tt>chainlength</tt>.
