# [Pw_forum] Problem in Variable-cell

Stefano de Gironcoli degironc at sissa.it
Thu Oct 28 00:28:46 CEST 2004

```On Thu, 21 Oct 2004, Varadharajan Srinivasan wrote:

Are you sure you are giving the atomic positions in crystal coordinates
in the scf run ?
Stefano de Gironcoli

> Dear users,
>
> I have been performing some variable-cell relaxation calculations using
> PWSCF on PbTiO3 and I come across an interesting problem. After having
> relaxed the unit cell I take the relaxed structure and perform an scf
> calculation on it to obtain again the forces, stress and energy that
> vc-relax gives me anyway. (I made sure that I chose the structure
> corresponding to the final energy and forces, ie. the structure reported
> before the final estimate) However, I obtain very different answers. The
> variable-cell gives me :
>
> ######################################################################################
>  new lattice vectors (alat unit) : ..........(input alat =       7.3627 (a.u.))
>   0.999624836   0.000000001   0.000000000
>   0.000000001   0.999624847   0.000000000
>   0.000000000   0.000000000   1.081152570
>  new unit-cell volume =    431.1954 (a.u.)^3
>  new positions in cryst coord
> Ti       0.499999997   0.500000003   0.535979348
> O        0.499999997   0.500000004   0.112901652
> O        0.499999997   0.000000004   0.614268219
> O       -0.000000003   0.500000004   0.614268218
> Pb      -0.000000002   0.000000003   0.004230374
>
> !    total energy              =  -332.37904437 ryd
>     estimated scf accuracy    <     0.00000000 ryd
>
>     convergence has been achieved
>
>     Forces acting on atoms (Ry/au):
>
>     atom   1 type  1   force =     0.00000000    0.00000000   -0.00000063
>     atom   2 type  2   force =     0.00000000    0.00000000   -0.00000168
>     atom   3 type  2   force =     0.00000000    0.00000000   -0.00000253
>     atom   4 type  2   force =     0.00000000    0.00000000   -0.00000253
>     atom   5 type  3   force =     0.00000000    0.00000000    0.00000738
>
>     Total force =     0.000008     Total SCF correction =     0.000008
>
>
>     entering subroutine stress ...
>
>          total   stress  (ryd/bohr**3)                  (kbar)     P=    0.01
>   0.00000008   0.00000000   0.00000000          0.01      0.00      0.00
>   0.00000000   0.00000008   0.00000000          0.00      0.01      0.00
>   0.00000000   0.00000000   0.00000002          0.00      0.00      0.00
>
>
>     Parrinello-Rahman Damped Dynamics: convergence achieved, Efinal=  -332.37904437
> #######################################################################################
>
> While from the corresponding structure the scf calculation gives me :
>
> #######################################################################################
> ibrav=  6, celldm(1) =7.359938, celldm(3)=1.081558
> Ti       0.499999997   0.500000003   0.535979348
> O        0.499999997   0.500000004   0.112901652
> O        0.499999997   0.000000004   0.614268219
> O       -0.000000003   0.500000004   0.614268218
> Pb      -0.000000002   0.000000003   0.004230374
>
> !    total energy              =  -332.94499390 ryd
>     estimated scf accuracy    <     0.00000000 ryd
>
>     band energy sum           =   -12.68025211 ryd
>     one-electron contribution =   -93.34743263 ryd
>     hartree contribution      =    73.14958559 ryd
>     xc contribution           =   -50.45891514 ryd
>     ewald contribution        =  -262.28823173 ryd
>
>     convergence has been achieved
>
>     Forces acting on atoms (Ry/au):
>
>     atom   1 type  1   force =     0.00000000    0.00000000   -0.00108980
>     atom   2 type  2   force =     0.00000000    0.00000000    0.00054152
>     atom   3 type  2   force =     0.00000000    0.00000000    0.00008334
>     atom   4 type  2   force =     0.00000000    0.00000000    0.00008334
>     atom   5 type  3   force =     0.00000000    0.00000000    0.00038160
>
>     Total force =     0.001281     Total SCF correction =     0.000005
>
>
>     entering subroutine stress ...
>
>          total   stress  (ryd/bohr**3)                  (kbar)     P= -288.66
>  -0.00195487   0.00000000   0.00000000       -287.57      0.00      0.00
>   0.00000000  -0.00195487   0.00000000          0.00   -287.57      0.00
>   0.00000000   0.00000000  -0.00197707          0.00      0.00   -290.84
> ##############################################################################
>
>
> Not only is the total energy very different but also the forces and the
> stress. I used a ecutwfc=40., ecutrho=240. in both calculations (and
> ecfixed=35. Ry in variable-cell) to be consistent in the cutoff energies.
>
> Is this discrepencancy to be expected or am I doing something wrong?
> I would greatly appreciate any help/suggestions.
>
> Thanks,
> Vardha.
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```