Dear Peter Blaha and Gavin Abo,

Non-overlapping muffin-tin spheres are used by WIEN2k and my LEED program 
eeasisss (Elastic Electron-Atom Scattering in Solids and Surface Slabs). But 
RMT(setrmt_lapw) is not automatically the best choice for RMT(eeasisss). LEED 
touching radii of atoms depend on exchange-correlation interaction between 
crystal electron gas (the WIEN2k electrons) and an incident LEED electron 
(energy 20-500 eV).

This is a N+1 electron scattering situation, where "N" signifies the WIEN2k 
electrons and "1" an alien LEED electron.

W2k can be reconciled with LEED using an atomic sphere approximation (ASA) 
extending into the Fourier expansion realm of W2k. A while ago you (P.B. and 
G.A.) suggested an ASA routine, in which I now use Poisson differentiation of 
vcoul_ASA in order to obtain clmsum_ASA. I consider the case LM=(0,0), 
sufficient for current LEED.

The considered structure is a supercell = a surface slab 15 layers thick, where 
layers 1-2 and 14-15 are C-O and O-C, respectively. Mirror symmetry about layer 
8. At the C-O layers vcoul_ASA(0,0) is continuous across the RMT radius, but 
clmsum_ASA(0,0) versus radius shows a step of the order of 10%.

Is the step k-point dependent? It does not seem so. With 16 and 48 k-points the 
clmsum_ASA(0,0) steps are preserved within 6 digits.

I shall be glad to supply the code. When the described numerical error is 
fixed, WIEN2k and eeasisss can be re-run self-consistently within the model of 
non-overlapping muffin-tin atoms.

Regards,
John Rundgren

KTH



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