  We investigate the physical properties of molecular hydrogen (H2) in isolated
and interacting disk galaxies with different masses and Hubble types by using
chemodynamical simulations with H2 formation on dust grains and dust growth and
destruction in interstellar medium (ISM). We particularly focus on the
dependences of H2 gas mass fractions (f_H2), spatial distributions of HI and
H2, and local H2-scaling relations on initial halo masses (M_h), baryonic
fractions (f_bary), gas mass fractions (f_g), and Hubble types. The principal
results are as follows. The final f_H2 can be larger in disk galaxies with
higher M_h, f_bary, and f_g. Some low-mass disk models with M_h smaller than
10^10 M_sun show extremely low f_H2 and thus no/little star formation, even if
initial f_g is quite large (>0.9). Big galactic bulges can severely suppress
the formation of H2 from HI on dust grains whereas strong stellar bars can not
only enhance f_H2 but also be responsible for the formation of H2-dominated
central rings. The projected radial distributions of H2 are significantly more
compact than those of HI and the simulated radial profiles of H2-to-HI-ratios
(R_mol) follow roughly R^-1.5 in MW-type disk models. Galaxy interaction can
significantly increase f_H2 and total H2 mass in disk galaxies. The local
surface mass densities of H2 can be correlated with those of dust in a galaxy.
The observed correlation between R_mol and gas pressure (R_mol ~ P_g^0.92) can
be well reproduced in the simulated disk galaxies.
