Curves of growth (COG) for the Cu i 327.3 nm transition. The dotted line to the left is the COG for the 3D model d3t63g40m20n01, the three solid lines to the right are those for the corresponding 1D_LHD model for three values of the microturbulent velocity: 0.5, 1.0 and 1.5  kms^-1, bottom to top. The dashed line is the 3D COG shifted arbitrarily by +0.58 dex along the x-axis. This highlights that the shape of the 3D COG differs from that of the corresponding 1D COGs and therefore the 3D correction depends on the EW of the transition.