  Recently, it has become possible to tune optical lattices continuously
between square and triangular geometries. We compute thermodynamics and spin
correlations in the corresponding Hubbard model using determinant quantum Monte
Carlo and show that the frustration effects induced by the variable hopping
terms can be clearly separated from concomitant bandwidth changes by a proper
rescaling of the interaction. An enhancement of the double occupancy by
geometric frustration signals the destruction of nontrivial antiferromagnetic
correlations at weak coupling and entropy $s\lesssim \ln(2)$ (and restores
Pomeranchuk cooling at strong frustration), paving the way to the long-sought
experimental detection of antiferromagnetism in ultracold fermions on optical
lattices.
