αs from the Hadronic Vacuum Polarisation

We present our result for the strong coupling constant computed from the u-d vector Hadronic Vacuum Polarisation function. We use n f = 2 + 1 flavours of Domain Wall fermions at 3 lattice spacings, generated by the RBC-UKQCD collaboration. We identify several possible pitfalls in this method for determining the coupling and illustrate how to resolve them.