[Wien] Force calculations: a problem with Bi

[mazin]

I am calculating a Bi-containing compound, BiOCuS. It has a rather 
simple tetragonal structure with two free parameters, Bi (z) and S (z).

When I vary Bi(z) keeping everything else fixed I obtain a well defined 
total energy minimum at one z, and a well defined Bu force zero at 
another z. Both sets of calculations are well converged, k-mesh 
convergence, RKmax, GKmax, fft, linearization parameters, rMT 
dependences etc are all tested. Including more semicores states does not 
affect the results either. A closer inspection shows that the force 
obtain by taking the derivative of the total energy agrees well with the 
calculated forces, if the latter are shifted by 63 mRy/bohr (a huge 
number). The calculations are nonmagnetic and scalar relativistic.

I vaguely remember having some problems quite some time ago with force 
calculations for another Bi compound, but at that time I never got to 
the bottom of that.

I wonder if this is a known problem and if anybody stumbled upon it before.

I.M.