[Pw_forum] vc-relax bug?

[Marcello Rosini]

Hallo everybody
I am using pw.x version 4.1 and I noticed a very strange behaviour when 
performing vc-relax calculation. I have done many tests, and I have 
reduced to the simplest case possible wich I illustrate here:

InAs fcc lattice:
* the lattice parameter scan and the murnaghan fit lead me to 11.177au 
lattice constant with total energy -16.91675895 Ry
*starting from 11.177 fcc lattice I perform a vc-relax, and I find a new 
unit cell with ~0.98% smaller lattice constant. All the symmetries are 
preseved and the InAs distance is still (1/4,1/4,1/4) of the new lattice 
parameter. Total energy is now -16.91886930 Ry
*I run a new scf calculation with the new lattice parameters and now I 
get Etot=-16.91485951... obviously higher, as it was or the murnaghnan 
calculation.

the question is:
is there something wrong in the vc-relax procedure? (you can find my 
input in the following)

thank you
Marcello

-----------------------

&control
    calculation = 'vc-relax'
    restart_mode='from_scratch',
    nstep=100,
    pseudo_dir = '~/AB_INITIO/PSEUDO/LDA',
    outdir='./'
    etot_conv_thr=1.0e-7,
    forc_conv_thr=1.0e-5,
/
&system    
      ibrav=2,
      celldm(1)=11.177,
      nat=2,
      ntyp=2,
      ecutwfc =15,
/
&electrons
    electron_maxstep = 300,
    diagonalization='david'
    mixing_mode = 'plain'
    mixing_beta = 0.7 
    conv_thr =  1.0e-8,
/
&IONS
    ion_dynamics='bfgs',
/
&CELL
    cell_dynamics='bfgs',
/
ATOMIC_SPECIES
In   114.818   in.cpi.UPF
As   74.922   As.pz-bhs.UPF
ATOMIC_POSITIONS
In      0.00 0.00 0.00
As      0.25 0.25 0.25
K_POINTS automatic
6 6 6 1 1 1


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  tel:   +39 059.205.5067
  email: marcello.rosini at unimore.it

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