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 Purpose: calculate degree of correlation
          between modes of a cpd file

  Usage:  cpdcorrelation
 [-h|-?|-ver] [-d -f }

    d/D:  Indicates a debug version
      P:  compiled with OpenMP support

 optn: -h         : Print this help-text.
       -?         : Print this help-text.
       -ver       : Version information.

 -e          : Relative compression error per mode,
                    default: err=1.d-5
 -d      : Read dvr from  rather
                    than from ./dvr
 -f      : Read cpd from  rather
                    than from ./cpd
 -no-spp          : Do not trafo to natural potentials
                    (i.e. no compression)
 -w               : Allow overwrite

 The program first calculates the entropies
 
   S = -sum_j p_j ln(p_j)
 
 Where p_j are the normalized natural populations (i.e.,
 sum_j p_j = 1) of one- and two-mode reduced density matrices.
 The correlation is then calculated as
 
   C(m,m1) = S(m) + S(m1) - S(m,m1)
 
 Where m and m1 are two different modes, respectively
 
 In order to reduce the size of the 2D matrices, the single
 mode basis on the primitive grid is first transformed
 to a natural potential basis with a relative error
 (see -e option). The relative error measure is:
 E = sum(relevant p_j)/sum(all p_j)
 
 Output file: cpd_correlation_sorted
 
 The program supports OpenMP (compile with -P option)
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