Stability of lattice QCD simulations and the thermodynamic limit

We study the spectral gap of the Wilson--Dirac operator in two-flavour lattice QCD as a function of the lattice spacing $a$, the space-time volume $V$ and the current-quark mass $m$. It turns out that the median of the probability distribution of the gap scales proportionally to $m$ and that its width is practically equal to $a/sqrt{V}$. In particular, numerical simulations are safe from accidental zero modes in the large-volume regime of QCD.