[Q-e-developers] inconsistent x-component of printed forces (qe 5.0)

[Lorenzo Paulatto]

On 8 March 2013 13:16, Nicola Manini <Nicola.Manini at mi.infm.it> wrote:
> Hi,
>                 in a simple pw calculation, the printed forces disagree with
> the successive displacements to achieve structural optimization.
>
> The attached input & output (isotropically strained bulk Si, 2 atoms
> primitive cell) at the first iteration gives the following
>
>      Forces acting on atoms (Ry/au):
>
>      atom    1 type  1   force =     0.07692026   -0.07692026   -0.07692026
>      atom    2 type  1   force =    -0.07692026    0.07692026    0.07692026

That's because the forces are printed in cartesian units, not in
crystal ones. and in cartesian units the coordinates of your atoms
are:
         1           Si  tau(   1) = (   0.0000000   0.0000000   0.0000000  )
         2           Si  tau(   2) = (  -0.2000000   0.2000000   0.2000000  )

Also, not that you are losing some symmetry operation because of
incompatibility with the FFT grid

cheers

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05