[Wien] basis set size for oxygen crystal

[Stefaan Cottenier]

Dear wien2k community,

I'm puzzled by the following observation about convergence tests for a 
molecular crystal built entirely from O2-molecules (two O2-molecules per 
unit cell): the basis set needs to be particularly large before even the 
the volume of the unit cell can be reliably determined.

Does anybody knows what is the mathematical/physical reason for this?

More details:

The RMT for oxygen is 1.02, dictated by the bond length in the O2 
molecule. The number of k-points was safely converged. For any basis set 
reported in the table hereafter, the E(V)-curves were nice and smooth -- 
only the position of the minimum and the curvature (bulk modulus) keep 
changing.

The table lists the RKMax value, the matrix size, the corresponding 
RKMax if the muffin tin radius would be 2.0 (that gives a better feeling 
of the large size of this basis set) and the volume of the unit cell 
determined from an equation of state fit:

4.38  574  8.6  487.7
4.78  750  9.4  506.4
5.20  945 10.2  502.4
5.60 1182 11.0  503.7
6.00 1426 11.8  505.0

There might be a numerical scatter of about 1-2 in the volumes given in 
the right-most column. This means that the difference between the last 
three volumes is probably negligible, and something as 504 au^3 is the 
converged volume. But the jumps from 487 to 506, and from 506 to 502 are 
definitely not due to numerical noise.

Why does this case require such a large basis set?

Thanks,
Stefaan