[QE-users] What's representation 0 in ph.x?

[Lorenzo Paulatto]

> start_irr = 235
> last_irr = 240

Hello,
I did not understant that the molecule was adsorbed on a surface! In 
this case it makes sense to compute several q-points (not orthogonal to 
the surface!) However, if you set start_irr and last_irr, you are only 
computing a few rows of the dynamical matrix. As the phonons are the 
square roots of the eigenvalues of this matrix, you need the entire 
matrix in order to compute them.

You can compute differents irreducible representations (irr) separately 
and then combine the results together as explained in the Grid_example 
of phonon. But you still need all the matrix. If you have reason to 
think that the CO vibration is completely decoupled from the rest of the 
system, you may try to get its vibrational frequency by doing finite 
differences. It would also be possible to modify phonon to only compute 
the minor of the dynamical matrix that involve CO, then diagonalize it, 
but it is not currently implemented.

hth

-- 
Lorenzo Paulatto - Paris