[Pw_forum] calculating cohesive energy

[Duy Le]

Hi,

I think you may need to increase the k-point mesh for bulk calculation and
do the spin-polarized for isolated atom. Another point: the calculated
cohesive energy in general depends very much not only on the functional but
also the pseudized potential (cutoff radius, configuration...). So one may
consider to calculate it with many different available potentials.

Best,

On Mon, Aug 31, 2009 at 2:54 PM, Stefano Baroni <baroni at sissa.it> wrote:

> No need to post twice the same message ...Don't now about your specific
> system, but it is not unusual that DFT overestimate binding energies (less
> so for PBE than LDA)
> Please, do not forget to sign your posts, including your affiliation
> SB
>
> On Aug 31, 2009, at 8:37 PM, udayagiri sai babu wrote:
>
> Hi all
> i am claculating cohesive energy of V using pwscf but i got -8.15 ev/atom
> against an experimental value of 5.3 ev/atom which is almost 50% off can
> anybody say what is going wrong with the calculation. the method i used is
> calculate the total energy of a bcc crystal with 1 atom and deduct the
> energy of isolated atom from it. the following input files are for finding
> the total energy for isolated atom and the atom in the crystal. Please tell
> me if my methodology is wrong.
>
> *cohesive energy=E per atom (in the crystal)-E per atom (isolated)*
>
> *input file for finding the total energy of an isolated atom*
>
> &control
> calculation = 'scf',
> restart_mode='from_scratch',
> outdir='/home/sai/temp',
> prefix='vanadium'
> pseudo_dir='/home/sai/softwares/espresso-4.0.1/pseudo/',
> tstress = .TRUE.,
> tprnfor = .TRUE.,
> /
> &system
> ibrav=1
> celldm(1)=30,
> nat=1,
> ntyp=1
> ecutwfc=25,
> ecutrho=100,
> occupations='smearing',
> smearing='mv',
> degauss=0.010000
> /
> &electrons
> conv_thr=1.D-8,
> diagonalization='david',
>   mixing_mode='plain',
> mixing_beta= 0.3
> /
> ATOMIC_SPECIES
> V  50.9415  V.pbe-n-van.UPF
> ATOMIC_POSITIONS (crystal)
> V       0.00000000      0.00000000      0.00000000
> K_POINTS {automatic}
> 1 1 1 0 0 0
>
> *input file for finding the total energy of the atom in the crystal*
>
> &control
> calculation = 'scf',
> restart_mode='from_scratch',
> outdir='/home/sai/temp',
> prefix='vanadium'
> pseudo_dir='/home/sai/softwares/espresso-4.0.1/pseudo/',
> tstress = .TRUE.,
> tprnfor = .TRUE.,
> /
> &system
> ibrav=3
> celldm(1)=5.654,
> nat=1,
> ntyp=1
> ecutwfc=25,
> ecutrho=100,
> occupations='smearing',
> smearing='mv',
> degauss=0.010000
> /
> &electrons
> conv_thr=1.D-8,
> diagonalization='david',
> mixing_mode='plain',
> mixing_beta= 0.3
> /
> ATOMIC_SPECIES
> V  50.9415  V.pbe-n-van.UPF
> ATOMIC_POSITIONS (crystal)
> V       0.00000000      0.00000000      0.00000000
> K_POINTS {automatic}
> 8 8 8 0 0 0
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>
>
>
>    ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /
> stefanobaroni (skype)
>
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-- 
--------------------------------------------------
Duy Le
PhD Student
Department of Physics
University of Central Florida.
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