  Based on the direct method of molecular dynamics, the temperature dependence
of energy, partition function, entropy and free energy, describing the
first-order phase transition, is calculated for the classical
Peyrard-Bishop-Dauxois model of the DNA molecule. It is shown that without
taking into account the quantum freezing of the degrees of freedom at low
temperatures, the relationship between these quantities is not determined. An
estimation was given for the number of bubbles in homogeneous DNA chains:
PolyA/PolyT and PolyG/PolyC.
