The two fasta files must be in the working directory. This function use zpaq to compress files. Thus, the zpaq software must be installed on your system and in the search path for executables if you wish to use this function.
NCD = (Z(xy) - min(Z(x), Z(y))) / max(Z(x), Z(y))
Where Z(x), Z(y) and Z(xy) are the lengths of the compressed versions of seq1, seq2 and the concatenated sequences 1 and 2, respectively.