[Pw_forum] ambiguous results with vc-relax

[Guda Karthik]

Hi,

    The total energy doesn't converge till 80 Ryd of ecutwfc. I am using Ni
and Ti USPPs (Ti.pbe-sp-van_ak.UPF, Ni.pbe-nd-rrkjus.UPF). My calculations
become very slow while using such a large kinetic energy cutoff. am I doing
something wrong here?

regards,
Karthik

On Tue, Aug 17, 2010 at 4:46 PM, Stefano de Gironcoli <degironc at sissa.it>wrote:

> please note that the two outputs that you submitted ARE NOT examples of
> vc-relaxation but of structural relaxation of the internal coordinates
> only. Your question is anyway legittimate...
> Are you sure to have converged the total energy of your system with
> respect to cutoff to a sufficient level ?
> when comparing  different volumes one is using different basis sets in
> the two cases. If convergence w.r.t. cutoff is not sufficient this  can
> lead to sudden decreases of the energy when increasing the volume.
>
> stefano
> SISSA and DEMOCRITOS
>
> Guda Karthik wrote:
> > Hi,
> >
> >     Here are the input and output files.
> >
> >     b19_Acklatpos_new.* - original structure
> >
> >     b19_pwscf.* - scf calculation with a structure obtained after
> vc-relax
> >
> >     Please have a look if they are of help.
> >
> > regards,
> > Karthik
> >
> > On Sat, Aug 14, 2010 at 6:27 AM, Paolo Giannozzi <giannozz at democritos.it
> >wrote:
> >
> >
> >> On Aug 4, 2010, at 4:41 , Guda Karthik wrote:
> >>
> >>
> >>> I am trying to relax an orthorhombic structure using vc-relax
> >>> (celldofree - xyz). I start with a structure which is at a Pressure
> >>> of -20.5 Kbar and at the end of the vc-relax I get a structure
> >>> which is at a pressure of -0.21 Kbar. Surprisingly, the structure
> >>> at pressure closer to zero is higher in energy than the original
> >>> structure by 8 * 10^-4 eV/formula unit. All the symmetry elements
> >>> have been conserved during relaxation and the initial and final
> >>> structures are the same. I am not able to make sense of this
> >>> result. I observe this inconsistency in my B2 structure as well.
> >>> Please help regarding this.
> >>>
> >> hard to help without input and output
> >>
> >> P.
> >> ---
> >> Paolo Giannozzi, Dept of Physics, University of Udine
> >> via delle Scienze 208, 33100 Udine, Italy
> >> Phone +39-0432-558216, fax +39-0432-558222
> >>
> >>
> >>
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> >>
> >>
> >
> >
> >
> >
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-- 
Karthik Guda
Graduate Student
Materials Engineering
Purdue University
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