  The impact of various types of external potentials to the Peyrard-Bishop DNA
denaturation is investigated through statistical mechanics approach. The
partition function is obtained using transfer integral method, and further the
stretching of hydrogen bond is calculated using time independent perturbation
method. It is shown that all types of external potentials accelerate the
denaturation processes at lower temperature. In particular, it is argued that
the Gaussian potential with infinitesimal width reproduces a constant force at
one end of DNA sequence as already done in some previous works.
