Scale setting for Nf=3+1 QCD

We present the scale setting for a newset of gauge configurations generated with Nf = 3+1Wilson quarks with a non-perturbatively determined clover coefficient in a massive O(a) improvement scheme. The three light quarks are degenerate, with the sum of their masses being equal to its value in nature and the charm quark has its physical mass. We use open boundary conditions in time direction to avoid the problem of topological freezing at small lattice spacings and twisted-mass reweighting for improved stability of the simulations. The decoupling of charm at low energy allows us to set the scale by measuring the value of the low-energy quantity t 0 /a2, which is the flow scale t0 at our mass point, and comparing it to an Nf = 2+ 1 result in physical units. We present the details of the algorithmic setup and tuning procedure and give the bare parameters of ensembles with two lattice spacings a = 0.054fm and a = 0.043fm. We discuss finite volume effects and lattice artifacts and present physical results for the charmonium spectrum. In particular the hyperfine splitting between the.c and J/. mesons agrees very well with its physical value.