[Pw_forum] unexpected results after relaxation

[Padmaja Patnaik]

Dear All

I am plotting density of states for unrelaxed and 
relaxed calculations of cubic SiC with B, Al and Ga as substitutional 
impurity. Substitutions are done for both Si and C sites separately. The
 density plot without relaxation looks OK. But after relaxation the 
density plot undergoes a drastic change. I am not able to understand 
whats going wrong. Here I am sending the density plots for all the cases
 with and without relaxation. In all the figures the top of valence band
 is taken as zero on the energy axis. Calculations are done with a 
supercell of 128 atoms with one single impurity atom in it. The dashed 
(purple)  line is the Fermi energy. The dotted line shows the calculated
 band gap. I am aslo giving here a portion of the relaxed input file 
used for calculations.

 &control
   
 calculation='relax'
    restart_mode='from_scratch',
    
prefix='sic',
    pseudo_dir = '/espresso-4.0.5/pseudo/',
    
outdir='/tmp/'
    tprnfor= .TRUE.
 /
 &system
    
ibrav=2, celldm(1) =32.9648, nat=128, ntyp=3,
    nspin = 2,
    
starting_magnetization(1)=0.0,starting_magnetization(2)=0.0,
    
starting_magnetization(3)=0.0,
    ecutwfc = 30.0,
    
occupations='smearing', smearing='methfessel-paxton', degauss=0.02
 /
 &electrons
   
 mixing_beta = 0.7
 /
&IONS
  pot_extrapolation = 
"second_order",
  wfc_extrapolation = "second_order",
 /
ATOMIC_SPECIES
  
 Si  28.086   Si.pz-vbc.UPF
   C 
 12.00     C.pz-vbc.UPF
   Al 26.98154  Al.pz-vbc.UPF

As ssen 
in figure after relaxation we found mane levels inside band gap at there
 is no gap for Al and Ga case. How is this possible? I have also done 
for few other impurities and in most of the cases the C site 
substitution after relaxation gives multiple levels inside band gap. Can
 anybody please clarify whats going wrong?

Thanks in advance.
regards

Padmaja Patnaik

Research Scholar

Dept of Physics

IIT Bombay

Mumbai, India
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