  Following a suggestion by Aoki and B\"ar, c2 can be extracted by analyzing
volume effects in 2-pion states. To this end we consider renormalized ratios of
four point to two point correlation functions. We present the results from
various CLS lattices, with pion masses ranging from 280 to 450 MeV and lattice
spacings of 0.07 fm and 0.08 fm. This low energy constant is useful to
understand discretization effects in chiral extrapolations with Wilson
fermions, especially for quantities which vanish in the chiral limit, like the
pion mass. Since our procedure is computationally cheap and straightforward, it
is recommended as a routine study for any Wilson-type simulation, as a check of
discretization effects.
