elves in the present magnetic cell! 
ans =
  -1.1400e-06
</pre><h2 id="6">Spin wave spectrum</h2><p>We calculate the spin wave spectrum and compare it to published results. Since the momentum transfer vector <b>Q</b> has opposite sign in SpinW and in the published PRL paper, we change the sign manually here. Also for direct comparison, we calculate the spin-spin correlation function in the coordinate system commonly used for polarised neutron scattering (x||Q, z perpendicular to the scattering plane). To de