  Finite-temperature properties of the frustrated Hubbard model are
theoretically examined by using the recently proposed thermal pure quantum
state, which is an unbiased numerical method for finite-temperature
calculations. By performing systematic calculations for the frustrated Hubbard
model, we show that the geometrical frustration controls the characteristic
energy scale of the metal-insulator transitions. We also find that entropy
remains large even at moderately high temperature around the region where the
quantum spin liquid is expected to appear at zero temperature. We propose that
this is a useful criterion whether the target systems have a chance to be the
quantum spin liquid or the non-magnetic insulator at zero temperature.
