  The mechanism of Mott transition in the Hubbard model on the square lattice
is studied without explicit introduction of magnetic and superconducting
correlations, using a variational Monte Carlo method. In the trial wave
functions, we consider various types of binding factors between a
doubly-occupied site (doublon, D) and an empty site (holon, H), like a
long-range type as well as a conventional nearest-neighbor type, and add
independent long-range D-D (H-H) factors. It is found that a wide choice of D-H
binding factor leads to Mott transitions at critical values near the band
width. We renew the D-H binding picture of Mott transitions by introducing two
characteristic length scales, the D-H binding length l_{DH} and the minimum D-D
distance l_{DD}, which we appropriately estimate. A Mott transition takes place
at l_{DH}=l_{DD}. In the metallic regime (l_{DH}>l_{DD}), the domains of D-H
pairs overlap with one another, thereby doublons and holons can move
independently by exchanging the partners one after another. In contrast, the
D-D factors give only a minor contribution to the Mott transition.
