[Pw_forum] Not getting right Band structure

Wed Jul 29 10:35:54 CEST 2009

Hi sir,

 Sir i have calculated the band structure of YVO4  and it is coming as near about 0.5 ev .. sir as suggested by you in earlier reply that 

>means band gap at gamma = 0.3058-(-8.9833)=  8.2891
>  which is very large ??

Not necessarily, the offset of band in a periodic system is arbitrary. You  
need to know where you Fermi energy is, or which are the HOMO and LUMO of  
your system to find the gap.

I have calculated Fermi energy and it is coming as 6.108. The band gap of YVO4 is 3.9 eV experimental and DFT calculates the band gap as nearly half i..e. it should come as 1.8 eV approx. Please help.. Thanks In advance.My Input files are listed below...

SCF INPUT
===============================================

 &control
    calculation='scf',
    restart_mode='from_scratch',
    prefix='yvo'
    pseudo_dir = '/home/physics/pseudo/',
    outdir='/home/physics/work/yvo/temp'
 /
 &system
    ibrav =1, a=7.118300514, b= 7.118300514, c=6.289300454
, nat =24, ntyp= 3,  ecutwfc = 40.0,ecutrho=300,
 occupations='smearing', smearing='mp', degauss=0.01, 

 /
 &electrons
    mixing_beta = 0.7
    diagonalization = 'david'
    conv_thr =  1.0e-6
/
 &ions
 /

ATOMIC_SPECIES
  Y     88.90585     Y.pbe-nsp-van.UPF
  V     50.9415      V.pbe-sp-van.UPF
  O     15.9994      O.pbe-van_ak.UPF
ATOMIC_POSITIONS {angstrom}
Y             0.000000000   5.338725356    0.786162553
/
/

K_POINTS {automatic}
  4 4 4 0 0 0
=======================================================
                               Bands Input
======================================================

 &control
    calculation='bands'
    pseudo_dir = '/home/physics/pseudo/',
    outdir='/home/physics/work/yvo/temp',
    prefix='yvo',

 /
 &system
    ibrav =1, a=7.118300514, b= 7.118300514, c=6.289300454, nat=24, ntyp=3, 
ecutwfc = 40.0,ecutrho=300, occupations='smearing', smearing='mp', degauss=0.01, nbnd= 115, 

 /
 &electrons
   mixing_beta=0.7
   conv_thr=1.0e-6
 diagonalization = 'david'
/
&ions
 /

ATOMIC_SPECIES
 Y       88.90585     Y.pbe-nsp-van.UPF
  V      50.9415      V.pbe-sp-van.UPF
  O     15.9994      O.pbe-van_ak.UPF
ATOMIC_POSITIONS {angstrom}
Y             0.000000000   5.338725356    0.786162553
\
\
K_POINTS
 26
0.000 0.000 0.000  1
0.000 0.000 0.125  1
0.000 0.000 0.250  1
0.000 0.000 0.375  1
0.000 0.000 0.500  1                             
0.125 0.125 0.500  1                             
0.250 0.250 0.500  1                             
0.375 0.375 0.500  1                            
0.500 0.500 0.500  1                             
0.500 0.500 0.333  1                             
0.500 0.500 0.166  1                             
0.500 0.500 0.000  1                             
0.375 0.375 0.000  1                             
0.250 0.250 0.000  1                             
0.125 0.125 0.000  1                             
0.000 0.000 0.000  1                             
0.000 0.000 0.166  1                            
0.000 0.000 0.333  1                             
0.000 0.166 0.500  1                           
0.000 0.333 0.500  1                            
0.000 0.500 0.500  1                             
0.000 0.500 0.333  1                             
0.000 0.500 0.166  1                             
0.000 0.500 0.000  1                             
0.000 0.333 0.000  1                             
0.000 0.166 0.000  1 
============================================================
===========================================================
 &inputpp
    prefix  = 'yvo'
    outdir = '/home/physics/work/yvo/temp'
    filband = 'yvobands.dat'
/
========================================
Plotband INPUTS
=======================================
yvobands.dat
-10  12
yvobands.xmgr
yvobands.ps
6.1088
1.0  0.00
================================================
============================================

With regards,
Neel Singh
Univeristy of Delhi
--- On Tue, 28/7/09, Lorenzo Paulatto <paulatto at sissa.it> wrote:

From: Lorenzo Paulatto <paulatto at sissa.it>
Subject: Re: [Pw_forum] Not getting right Band structure
To: "PWSCF Forum" <pw_forum at pwscf.org>
Date: Tuesday, 28 July, 2009, 10:07 AM

In data 28 luglio 2009 alle ore 11:54:14, Neel Singh  
<neelphysics at yahoo.in> ha scritto:
> means band gap at gamma = 0.3058-(-8.9833)=  8.2891
>  which is very large ??

Not necessarily, the offset of band in a periodic system is arbitrary. You  
need to know where you Fermi energy is, or which are the HOMO and LUMO of  
your system to find the gap.

> 2) In the output of SCF and Bands , the output file has 4 warnings

This a very frequently asked question, please see the corresponing entry  
in the manual:
http://www.pwscf.org/guide/2.1.2/html-node/node56.html
http://www.pwscf.org/guide/2.1.2/html-node/node57.html

> for this I have already increase the following quantities  in the   
> increments
> kinetic-energy cutoff     =      40.0000  Ry to 60
>      charge density cutoff     =     250.0000  Ry to 400
> But still the warnings are there.

It is not a big increment of the cutoff that will solve this problem, but  
a small increment or nr1, nr2 nr3

cheers, and good work

-- 
Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
skype: paulatz
www:   http://people.sissa.it/~paulatto/

     *** save italian brains ***
  http://saveitalianbrains.wordpress.com/
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