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The energy derivative test is invoked by the `DERIV` keyword.
The code works by computing numerical
derivatives of energy as shown:
dU(x_i)/dx_i (x_i) = (U(x_i+h) - U(x_i-h)) / 2h
and comparing them against the analytic
derivatives computed by the code.
Statistics of the differences are
calculated and a histogram of the differences is displayed.

The keyword, `DELTA`, specifies the value of *h* in the above
expression. The default value is 0.001. For single precision
arithmetic, this value is close to optimum.^{1} The keyword, `CUT`,
specifies a printing cutoff. Any difference in derivative whose
magnitude exceeds this value will be individually displayed.

[1] See S.D. Conte and
C. de Boor, *Elementary Numerical Analysis, an Algorithmic Approach*,
McGraw-Hill Book Company, New York 1972, p. 282