def METHOD_NAME(a, b, dims=None):
    # c[x] = (1/vol) sum_y a[y]*adj(b[y+x])
    F = gpt.fft(dims=dims)
    if dims is not None:
        norm = numpy.prod([a.grid.gdimensions[d] for d in dims])
    else:
        norm = a.grid.fsites
    return F(gpt(float(norm) * F(a) * gpt.adj(F(b))))