[Pw_forum] Problems with integrated charge for a large supercell relaxation

[JR Schmidt]

I am having problems with a calculation repeatedly aborting during a 
supercell relaxation.  After MANY successful (force / displacement) 
iterations, and when the force on the atoms is quite small (i.e. near 
convergence of the optimization), the calculation aborts with the error:

WARNING: integrated charge=   405.28031513, expected=   408.00000000

 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     from electrons : error #         1
     charge is wrong
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

I checked the archives, and this problem has come up before for 
supercell calculations.  I realize that the most obvious explanations are:

(1)  Bad geometry
(2)  Bad cutoffs

With regards to (1), I have essentially ruled out by:
 * Checking the geometry in xcrysden.
 * Noting that if I restart the relaxation from the final geometry of 
the failed relaxation, the resulting SCF converges just fine, and the 
optimization continues (although the approximate Hessian is lost, so the 
optimization does not proceed very well).

With regards to (2), I am using an ultrasoft PP.  For ectuwfc I am using 
30 Ry, which works fine for all the SCF calculations.  I tried 
increasing ecutrho from 200 Ry to 300 Ry (which I think is quite 
conservative) to no avail.

Is there something happening in the course of the optimization (say, for 
example, an atom moving from one side of the unit cell to another across 
the boundary?) that would render the previous charge density a horrible 
guess, and cause the optimization to bomb out?

I don't think the optimization itself is leading to unreasonable 
geometries, as I looked at the final result (right before the crash); it 
looks fine.  Also, as I mentioned, I can successfully restart 
('from_scratch', but with the final geometry) the optimization from that 
point.

Thanks for any guidance.

-- 
J.R. Schmidt
Assistant Professor of Chemistry
Room 8305D
Department of Chemistry
University of Wisconsin-Madison
1101 University Ave
Madison, WI 53706

Phone: (608) 262-2996
Fax: (608) 262-9918
E-mail: schmidt at chem.wisc.edu
http://www.chem.wisc.edu/people/profiles/schmidt.php