  The short supercoiled circular DNA molecules are shown to be glassy systems
and canonical Metropolis Monte Carlo simulations of the systems tend to get
stuck in local metastable energy basins. A novel Monte Carlo algorithm is
developed to alleviate the problem of ``ergodicity breaking'' of the glassy
systems, in which the Markov process is driven by an explicitly analytic weight
factor with enhanced probability in both low- and high-energy regions. To
characterize the degree of puckering of the supercoiled DNA loops, a new
quantity of aplanarity is introduced as the shortest principal axis of
configurational ellipsoid of DNA. With the suggested Monte Carlo method, the
quantitative correlation between supercoiling degree and buckling of DNA is
attained. With supercoiling stress increasing, the conformational transition
from a circle to mono-, diplo- or triple interwound superhelical structure will
take place in a successive but decreasingly abrupt mode.
