[Wien] Converging Gmax

[David Tompsett]

Dear All,

I have a query about converging energies with respect to Gmax. I am 
working on a d-metal metallic system.

I have been attempting to converge the difference in total energy 
between a non-magnetic and antiferromagnetic spin configuration. The 
calculation is done with the GGA using RKmax=9 and RMT's of 2.1 and 
8000Kpt in the full Brillouin zone.

With these parameters (which I tested for convergence) I have then tried 
to converge the energy difference with respect to Gmax and found some 
behaviour that I can not understand. All calculations were converged 
with -ec 0.00001.

For the non-magnetic calculation the energies go as:
Gmax        Energy
12            -28765.981815
14            -28765.984603
16            -28765.983740
18            -28765.983085
20            -28765.982506
22            -28765.982092
24            -28765.981842
26            -28765.981561

So this indicates large changes of order of 0.01Ry as Gmax is changed. 
This seems like an extraordinarily high Gmax for convergence

In contrast for the antiferromagnetic calculation we find little 
variation as we change Gmax:
Gmax         Energy
12            -28765.982610
14            -28765.982595
18            -28765.982645
26            -28765.982645

 From this I have a few questions:
1) Why would a non-magnetic calculation take such a larger Gmax to 
converge? Is Gmax=26 reasonable?

2) Why might the convergence of the AFM calculation be much faster in Gmax?

3) When converging Gmax is it acceptable to reuse the charge density 
(*.clm*) files from the previous calculation? ie. should I run dstart 
again before each new value of Gmax?

Thank you for any help,
David.

-- 
David A. Tompsett
Quantum Matter Group
Cavendish Laboratory
J. J. Thomson Avenue
Cambridge CB3 0HE
U.K.
Tel: +44 7907 566351 (mobile)
Fax: +44 1223 768140
http://www-qm.phy.cam.ac.uk/