[QE-users] [SUSPICIOUS MESSAGE] Re: Minimum lattice parameter calculation for Manganese

[Giuseppe Mattioli]

Dear Mash
When an scf calculation does not converge the problem is almost always  
in the starting structure. In your case, you have used the lattice  
parameter of alpha-Mn, which has more than 1 atom inside the unit  
cell. See here, for example to look at the crystal structure

http://www.crystallography.net/cod/9011068.html

Your Mn atoms are therefore quite far from one another (around 7.7 A)  
and their density does not overlap, causing your convergence  
difficulties. As bcc is not the stable structure of Mn you should  
guess a better (shorter) starting lattice parameter. Moreover, You may  
also want to consider the fact that bcc Mn can be paramagnetic,  
depending on the lattice parameter. You should therefore use nspin=2  
and a meaningful value for the starting magnetization. Finally, when  
you use occupations='smearing' the pw.x code add a suitable number of  
unoccupied bands to ensure the stability of the calculation for  
metallic systems. nband=9 might be on the scarce side: if you skip the  
variable you automatically obtain 12 bands, which should ensure a  
better convergence (with a meaningful lattice parameter...)
HTH
Giuseppe

Quoting Mashroor Nitol <mashroornitol at gmail.com>:

> Dear Users:
>
> I am new in QE. I want to know for which lattice parameter Manganese will
> have minimum potential energy.  For this the following input file is used.
> But every time same error message is showing "  convergence is NOT achieved
> after 100 iterations: stopping. ". Can anyone help me out ?
>
>
> !!!!!!!!!!!!!!!!!!!!!!!!!!!        INPUT    !!!!!!!!!!!!!!!!!!!!!
>
>  &control
>     calculation='vc-relax'
>     prefix='Mn_bcc_8'
>     restart_mode='from_scratch'
>     wf_collect = .false.
>     pseudo_dir = '/home/mash/Desktop/qe/qe-6.3/pseudo'
>     outdir= 'Mn_bcc_8/temp'
>     tprnfor = .true.
>     tstress = .true.
>     etot_conv_thr = 1.0E-5  ,
>     forc_conv_thr = 1.0D-4 ,
>  /
>  &system
>     ibrav=  3
>     A = 8.9125
>     nat=  1
>     ntyp= 1
>     ecutwfc = 37.0
>     ecutrho = 250.0
>     nbnd = 9
>     occupations='smearing'
>     smearing='methfessel-paxton'
>     degauss=0.005
>  /
>  &electrons
>    conv_thr=1d-7
>  /
>  &ions
>  ion_dynamics = 'bfgs'
>  /
>  &CELL
>  cell_dynamics = 'bfgs'
>  cell_dofree = 'all'
>  /
> ATOMIC_SPECIES
>  Mn 54.938044  Mn.pbe-spn-kjpaw_psl.0.3.1.UPF
> ATOMIC_POSITIONS {angstrom}
>  Mn 0.0 0.0 0.0
> K_POINTS automatic
>    8 8 8 0 0 0
>
> Thank you
>
> Mash
> Mississippi State University
>
> ________________



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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E-mail: <giuseppe.mattioli at ism.cnr.it>