[Pw_forum] phonon calculations for molecules

[Ruben Soria-Martinez]

Dear Quantum Espresso users, 

I'm attempting some basic
calculations in order to obtain the IR spectra of molecules. I did a
test for SiH4 molecule using the example given by Paolo in
http://www.fisica.uniud.it/~giannozz/QE-Tutorial/tutorial_advph.html.


On the other hand, I tried to do the same kind of simulation but now
centering the molecule at the center of a simple cubic lattice. 

 I saw
that centering the molecule at the center of the lattice I obtained some
imaginary frequencies, 6 normal modes that are zero and positive
frequencies (It's attached the file). 

I was wondering if I have to use
the transformation matrix in order to obtain the cartesian coordinates
(with the center of mass of the molecule at the origin of the cell) into
crystallographic units taking into account the symmetry of the lattice.


My question is, how is the way to define a molecule in order to obtain
the correct results? 

I know that centering the molecule at the center
of the lattice is wrong due to the symmetry (C1). 

Thanks in advance


Regards 

--
Rubén Soria Martinez

PhD Student

Insitut de Physique et
Chimie des Matériaux de Strasbourg (IPCMS).

23 Rue du Loess 
STRASBOURG

67034 France

TEL: +33 (0) 769289329s 
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