[Pw_forum] Convergence problem with Pb180Se180 Cluster optimization

[Giuseppe Mattioli]

Dear Naren
Your calculation is very expensive, and calculations of 0D objects in a plane-wave/PBC framework are often prone to convergence issues. This said, you 
can try to

1) perform a simple calculation of the PbSe unit cell with the same pseudopotentials. Are there similar convergence problems? You are using hgh that 
are norm-conserving pseudopotentials (NCPP) and they could require a richer pw basis set. I would not be surprised if, after proper convergence tests 
performed on the unit cell, you ended up with ecutwfc=70~80 Ry. There is no need to specify ecutrho, anyway. It is fixed to a default ecutwfc*4 value 
in the case of NCPP.

2) lower mixing_beta, even down to very small values such as 0.05~0.01. Sometimes it helps to reach convergence (slowly but steady).

It is impossible to say more without looking at you cluster, but if it is simply cut out from the bulk along low-index directions (e.g., a cube) you 
have to deal with a mess of surface dangling bonds that want to play nasty charge displacements between occupied and unoccupied electronic levels, 
even inducing a metallic character in your cluster. People used to saturate such dangling bonds with pseudo-H atoms (H atoms having formal charge 
lower or higher than 1.0, depending on the formal valence of the saturated atom). If you don't want to saturate the surface you must keep into account 
the possibility that the cluster surface reconstruct, and you should anticipate reconstruction trends by lplacing the atoms nearer to the expected 
final positions. It could be a very difficult task, look at the instructive story of the Si 7x7 reconstruction in this movie! 

https://www.youtube.com/watch?v=BXdC0NhAMBY

HTH
Giuseppe

On Friday, April 21, 2017 04:06:06 AM Narendranath Ghosh wrote:
> Dear all!
> 
> 
> 
> I am currently trying to optimizing a Pb180Se180 cluster using  QE.
> 
> But after 83 iterations it didn’t get converged.
> 
> *Input  *
> 
> calculation='relax',
> 
>   outdir='/staging/op/NAREN_USC/PbSe_360_FINAL_based_Pb68Se68/OUTPUT',
> 
>   prefix='PbSe-360',
> 
>   pseudo_dir='/staging/op/NAREN_USC/PbSe_360_FINAL_based_Pb68Se68/pseudo',
> 
>   verbosity='low',
> 
> /
> 
> 
> 
> &SYSTEM
> 
>   ibrav=0,
> 
>   celldm(1)=90.7068539791d0,
> 
>   nat=360,
> 
>   ntyp=2,
> 
>   ecutwfc=30.0d0,
> 
>   ecutrho=120.0d0,
> 
>   input_dft='PBE',
> 
>   occupations='smearing',
> 
>   smearing='gaussian',
> 
>   degauss=0.002000d0,
> 
> /
> 
> &ELECTRONS
> 
>   diagonalization='david',
> 
>   mixing_mode='plain',
> 
>   mixing_beta=0.700d0,
> 
> /
> 
> &IONS
> 
> /
> 
> ATOMIC_SPECIES
> 
>  Pb 207.200000d0 Pb.pbe-hgh.UPF
> 
>  Se 78.960000d0  Se.pbe-hgh.UPF
> 
> 
> 
> ATOMIC_POSITIONS {alat}
> 
>   Pb   0.4831100000d0   0.4740500000d0   0.4999300000d0
> 
>   Se   0.5190800000d0   0.5278700000d0   0.5981100000d0
> 
>   Se   0.5901900000d0   0.4712600000d0   0.2154800000d0
> 
> ……………………………………………………………….
> 
> K_POINTS {gamma}
> 
> 
> 
> CELL_PARAMETERS {alat}
> 
>   1.000000000000d0  0.000000000000d0  0.000000000000d0
> 
>   0.000000000000d0  1.000000000000d0  0.000000000000d0
> 
>   0.000000000000d0  0.000000000000d0  1.000000000000d0
> 
> 
> 
> 
> 
> *Output*
> 
>     iteration # 81     ecut=    30.00 Ry     beta=0.70
> 
>      Davidson diagonalization with overlap
> 
>      WARNING:     9 eigenvalues not converged in regterg
> 
>      ethr =  4.62E-04,  avg # of iterations = 21.0
> 
> 
> 
>      total cpu time spent up to now is    92680.3 secs
> 
> 
> 
>      total energy              =    2098.58547221 Ry
> 
>      Harris-Foulkes estimate   =  -16225.87153481 Ry
> 
>      estimated scf accuracy    <  209398.02191142 Ry
> 
> 
> 
>      iteration # 82     ecut=    30.00 Ry     beta=0.70
> 
>      Davidson diagonalization with overlap
> 
>      WARNING:     5 eigenvalues not converged in regterg
> 
>      c_bands:  5 eigenvalues not converged
> 
>      ethr =  4.62E-04,  avg # of iterations = 20.0
> 
> 
> 
>      total cpu time spent up to now is    95793.4 secs
> 
> 
> 
>      total energy              =    1914.32044477 Ry
> 
>      Harris-Foulkes estimate   =  -18801.41916562 Ry
> 
>      estimated scf accuracy    <  243341.55571758 Ry
> 
> 
> 
>      iteration # 83     ecut=    30.00 Ry     beta=0.70
> 
>      Davidson diagonalization with overlap
> 
>      WARNING:   138 eigenvalues not converged in regterg
> 
>      ethr =  4.62E-04,  avg # of iterations = 27.0
> 
> 
> 
>      total cpu time spent up to now is    99442.6 secs
> 
> 
> 
>      total energy              =    1659.36438352 Ry
> 
>      Harris-Foulkes estimate   =  -15989.48139862 Ry
> 
>      estimated scf accuracy    <  138246.99363207 Ry
> 
> 
> 
> Any comment to help me getting started is very much appreciated!
> 
> 
> 
> Best regards,
> 
> Naren
> 
> 
> 
> 
> 
> 
> 
> *Dr. Narendra Nath Ghosh*
> 
> *Research Associate*
> 
> *University of Gour Banga*
> 
> *Department of Chemistry*
> 
> *Malda-732102*
> 
> *India*
> 
> *Phone: +919126667601*

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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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