[Pw_forum] ambiguous results with vc-relax

[jia chen]

I did similar thing before, total energy doesn't need to converge...

On Mon, Aug 23, 2010 at 2:22 AM, Guda Karthik <karthik.guda at gmail.com> wrote:
> 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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-- 
Jia Chen

Dept of Chemistry
Princeton University
Princeton, NJ 08544