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[Xavier Gonze <xavier.gonze@uclouvain.be>]

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