A 3x3 matrix containing the derivative of the <tt>VIRIAL</tt> matrix
with respect to strain <tt>e_xx = e_11</tt>.

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<tt>DVIRIALDexx_ij</tt> = d(<tt>VIRIAL_ij</tt>) / (d<tt>e_11</tt>)

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<tt>VIRIAL</tt> is the Virial stress tensor and <tt>e</tt> is the strain tensor.

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Also see <tt>Dexx</tt>.
