  We investigate electron correlation effects in
internuclear-distance-dependent enhanced ionization of $\mathrm{H}_2$,
$\mathrm{LiH}$, and $\mathrm{HF}$ molecules by intense near-infrared laser
pulses using a 3D description of the systems with the time-dependent
generalized-active-space configuration-interaction method. This method
systematically incorporates electron-electron correlation of the quantum
many-electron system under consideration. Our correlated description of
diatomic molecules shows that enhanced ionization occurs at certain critical
internuclear separations and electron correlation systematically improves the
ionization probability in this process until convergence is reached. We
demonstrate the failure of the single-active-electron and the
configuration-interaction singles approximations to produce the correct
internuclear position and probability of the strong-field enhanced-ionization
process. We elucidate the role of low-lying electronic excited states in the
enhanced ionization process of diatomic molecules. There is clear evidence that
an accurate description of low-lying electronically excited states is important
to describe the non-perturbative enhanced ionization phenomenon in the
ultrashort intense near infrared laser pulses.
