  We have analyzed the dependence of average ground state energy per monomer,
$e$, of the complex of two random heteropolymers with quenched sequences, on
chain length, $n$, in the ensemble of chains with uniform distribution of
primary sequences. Every chain monomer is randomly and independently chosen
with the uniform probability distribution $p=1/c$ from a set of $c$ different
types A, B, C, D, .... Monomers of the first chain could form saturating
reversible bonds with monomers of the second chain. The bonds between similar
monomer types (like A--A, B--B, C--C, etc.) have the attraction energy $u$,
while the bonds between different monomer types (like A--B, A--D, B--D, etc.)
have the attraction energy $v$. The main attention is paid to the computation
of the normalized free energy $e(n)$ for intermediate chain lengths, $n$, and
different ratios $a=\frac{v}{u}$ at sufficiently low temperatures when the
entropic contribution of the loop formation is negligible compared to direct
energetic interactions between chain monomers and the partition function of the
chains is dominated by the ground state. The performed analysis allows one to
derive the force, $f$, which is necessary to apply for unzipping of two random
heteropolymer chains of equal lengths whose ends are separated by the distance
$x$, averaged over all equally distributed primary structures at low
temperatures for fixed values $a$ and $c$.
