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["Josef W. Zwanziger" <jzwanzig@jzwanzig.org>]

Thanks everyone for all your replies, it's now (pretty) clear to me how to deal with ecutsm and optcell.

 

Josef W. Zwanziger
Professor of Chemistry
Canada Research Chair in NMR Studies of Materials
Director, Atlantic Region Magnetic Resonance Centre
Department of Chemistry
Dalhousie University
6274 Coburg Road
Halifax, Nova Scotia B3H 4J3 Canada
tel: +1.902.494.1960
fax: +1.902.494.1310
web: http://jwz.chem.dal.ca
jzwanzig@jzwanzig.org,jzwanzig@dal.ca

----- Original Message ----
From: Xavier Gonze <xavier.gonze@uclouvain.be>
To: forum@abinit.org
Sent: Thursday, October 16, 2008 10:47:52 AM
Subject: Re: [abinit-forum] Questions about optcell /= 0 and ecutsm

Dear Joe,

On 16 Oct 2008, at 15:18, Josef W. Zwanziger wrote:

Yes, the same thing happens with NC's. I just re-ran Si with an opium-generated RRKJ NC pseudo, and when I optimize with dilatmx 1.1 and ecutsm 0.5, I get a final pressure of about 1E-6 GPA, if I then use the new acell and leave dilatmx and ecutsm in place, but do only an SCF run, I also get about 1E-6, but if I delete dilatmx and ecutsm from the input file, so they take their defaults, the pressure goes to 1E-2.

I also tried the same runs at a rather small ecut (for the first set I used ecut = 20 which gives very good convergence for Si, while in the second set I tried ecut = 5). In this case, the cell optimizes to 1E-7 GPa, re-running as scf with ecut and dilatmx non-default gives 1E-5, but deleting ecutsm and dilatmx gave -11.8 GPa.

Results with ecut = 40 are similar to ecut = 20; 1E-5 GPa for opt and for scf with dilatmx and ecutsm, 1E-3 for scf without dilatmx and ecutsm.

What you observe seems coherent, and corresponds to what is to be expected. Indeed :

- setting dilatmx to 1 or 1.1 does not really change the basis used for the wavefunctions,

but only the memory that is booked for storing the coefficients ;

- on the other hand, the use of ecutsm has an effect similar to a change of the basis set, even if ecut is the same, and this explains

 that if you restart a calculation with different ecutsm, the optimization of cell parameter is not exactly the same, hence giving

 non-zero pressure ;

- the effect of using ecutsm is smaller for larger ecut. This explains why you only get 1E-3 if you start from ecut=40 , while the corresponding

 value for ecut=20 is  1E-2, and for ecut=5 is 11.8 !

The best is to stay coherent, and, e.g. always use ecutsm 0.5 .

You should have a look to 

Bernasconi et al, J. Phys. Chem. Solids 56, 501 (1995)

G.-M. Rignanese et al. Phys. Rev.. B 52, 8160-8178 (1995) 

Best regards,

Xavier