[Pw_forum] Negative pressure during vc-relax

[Elie Moujaes]

Dear All,
 
I am trying to vc-relax a supercell made of 60 Carbon atoms. I have realized that during the iterations, the pressure is getting more negative. For example:
 
Total Force                 Pressure (kbar)
 
3.042645                    -179.66
1.543265                    -189.02
0.871984                    -193.18 
0.686871                    -194.73
0.537735                    -195.49
0.421031                    -195.96
0.366043                    -196.10
 
 
 Is that due to the choice of the pseudopotential, the convergence thresholds or maybe both? I will post a part of the input. Thanks in advance.
 
 

&control
    prefix='GB mgraphene',
    calculation='vc-relax',
    restart_mode='from_scratch',
    pseudo_dir = '/exp/home/caiapo/emoujaes/espresso/espresso-4.1.3/pseudo/',
    outdir='/exp/home/caiapo/emoujaes/espresso/Moujaes-Results/',
    etot_conv_thr = 1.0D-2,
    forc_conv_thr = 1.0D-2,
    tstress = .true.,
    tprnfor = .true.,      
 /
 &system    
    ibrav= 0, celldm(1) =1.889725989, nat=60, ntyp= 1, ecutwfc =30.0, ecutrho = 120.D0, occupations='smearing', smearing='mp', degauss=0.01
/
 &electrons
    conv_thr=1.D-2,     
    mixing_beta=0.7D0,
    diagonalization='david' 
 /
  &ions
    ion_dynamics = 'bfgs',
   /
  &cell
    cell_dynamics = 'damp-w',
    /
 
CELL_PARAMETERS hexagonal
24.52769   0.00000   0.00000
0.000003   6.614491  0.00000
0.000000   0.000000  10.0000
 
     
ATOMIC_SPECIES
 C  12.0107  C.pz-vbc.UPF
ATOMIC_POSITIONS angstrom
 C   -11.737980    -3.561278     0.000000
 C   -11.057718    -1.211571     0.000000
 C   -12.450000    -7.189838     0.000000
 C   -12.450000    -5.773759     0.000000
 C   -11.329191    -4.926800     0.000000
 C   -10.692399    -2.603942     0.000000
...................
 
As you can see I have used a convergence threshold for electrons of the order of 10^-2 (maybe it should be 10^-5??). The convergence on the forces is 10^-2 which I think is fine. the reason for putting such convergence thresholds is to gain a bit of time especially that i do not think one needs to go further down (than 10^-2  forces wise). Maybe also I am using an inconveninet pseudopotential..Any suggestions...
 
Elie Moujaes
University of Nottingham
Nottingham
NG7 2RD
UK
 		 	   		  
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