A 3x3 matrix (&Sigma;) that is needed to compute <tt>GH</tt> in Parrinello-Rahman barostat.

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Reference: Parrinello and Rahman, J. Appl. Phys. 52, 7182 (1981), Eq.(2.24).

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Also see <tt>GH</tt>, <tt>relax_freebox</tt>.
