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["Josef W. Zwanziger" <jzwanzig@dal.ca>]

Thanks for your response, Matthieu. Indeed, the symmetry-breaking effects
seem to accumulate in acell during the optimization run, the values in xred
appear to stay rigorously proper (that is, values that should be equal
actually are equal, to all digits, while in acell, in my case all values
should always be equal but they drift away from each other). I will look
further at the output and see if there are more clues.

Josef W. Zwanziger                                 
Prof. of Chemistry
Canada Research Chair in NMR Studies of Materials
Director, Atlantic Region Magnetic Resonance Centre
Dalhousie University
Halifax, NS B3H 4J3 
Canada

tel: 902.494.1960     net: jzwanzig@dal.ca
fax: 902.494.1867     web: http://jwz.chem.dal.ca


-----Original Message-----
From: Matthieu Verstraete [mailto:mjv500@york.ac.uk] 
Sent: August 27, 2007 10:10 AM
To: forum@abinit.org
Subject: Re: [abinit-forum] Cell optimization and symmetry breaking



No, there are no hidden options. I have observed this as well. Abinit 
symmetrizes the forces and stresses before applying them, so I don't know 
how it manages to break them in the end. Maybe some accumulated round-off. 
Of course it keeps the original symops from the initial structure, even if 
they are broken somehow. It would be nice to see when and where it 
appears. It might even be necessary to re-symmetrize the positions 
periodically (or at every step - it's cheap).

In my case it happens progressively (angles deviating from 90 deg etc). 
Same for you? Does it happen if you don't relax the cell? I doubt it - 
probably the stresses are to blame. If you could do a thorough search or 
look at the last decimals of the stresses that are applied (maybe beyond 
what is printed in the normal output) that would be great. In the Broyden 
algorithm they are also piped through the Hessian, so the numerical error 
could appear there. Dunno, maybe the Hessian itself has to be symmetrized.

Bye!

Matthieu

-- 
================================================================
Dr. Matthieu Verstraete                 mailto:mjv500@york.ac.uk
Dept. of Physics, University of York,     tel: +44 1904 43 22 08
Heslington, YO10 5DD York, United Kingdom fax: +44 1904 43 22 14