  The interaction-induced metal-insulator transition should be in the Ising
universality class. Experiments on layered organic superconductors suggest that
the observed critical endpoint of the first-order Mott transition belongs
instead to a different universality class. To address this question, we use
dynamical mean-field theory and a cluster generalization that is necessary to
account for short-range spatial correlations in two dimensions. Such
calculations can give information on crossover effects, in particular quantum
ones, that are not included in the simplest mean-field. In the cluster
calculation, a canonical transformation that minimizes the sign problem in
continuous-time quantum Monte Carlo calculations allows us to obtain very
accurate results for double occupancy. These results show that there are
important subleading corrections that can lead to apparent exponents that are
different from mean-field. Experiments on optical lattices could verify our
predictions.
