[Pw_forum] Surface Phonon and vib. of molecule

Stefano Baroni baroni at sissa.it
Sat Jul 17 13:19:27 CEST 2004

On Jul 17, 2004, at 1:08 PM, ashima wrote:

> Hi all,
> I'd like to calculate vibrational properties
> of crystal surfaces and molecules.
> But I don't have the knowlege about the way to calculate the surface 
> phonon.
> So, I have the following two questions:
> (1)
> Can ph.x code calculate phonon of slab model ?.
> If, 'Yes', please teach the methods.

as a first approximation, the computer does not know the difference 
between a "slab" and a "crystal". A "slab" is simply a crystal with 
lots of empty space stuffed into it. So, just set the supercell and 
calculate its vibrational modes "as though" it were a physical crystal. 
Of course, the number of modes thus obtained is limited by the 
thickness of the slab. When you're done with this excercise, please 
revert to this forum and we will teach you how to simulate a 
"semi-infinite" slab by patching interatomic force constants calculated 
for the bulk onto the results obtained for the finite slab.

> (2)
> Can ph.x code calculate vibrational modes of molecule, such as Benzene 
> ring ?.
> Does it needs a special manners ?.

Same as above. Just build a fake crystal with a benzene ring at the 
origin (say) of the unit cell, and lots of empty space surrounding it 
(i.e. choose the size of the unit cell large enough so that the charge 
densities of neighboring molecules do not overlap).

Hope this helps
Stefano Baroni

Stefano Baroni    ---  SISSA  &  DEMOCRITOS National Simulation Center
via Beirut 2-4 34014 Trieste Grignano / [+39] 040 3787 406 (tel) -528 

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