quantum-espresso/PHonon/examples/example12

This example shows how to use pw.x and phcg.x to calculate the normal modes 
of a molecule (SiH4). phcg.x can calculate only phonon modes at q=0, only if
 the Gamma point (k=0) is used to sum over the Brillouin Zone.

1) make a self-consistent calculation at Gamma.
   (input=sih4.scf.in, output=sih4.scf.out)
   Note that you need to specify K_POINTS {Gamma} in order to use 
   Gamma-specific algorithms (i.e. exploit the fact that psi(r) at k=0 
   are real to keep half of the plane waves and to perform two FFT's at
   the same time). If you use the alternative syntax, for instance:
   K_POINTS
   1
   0. 0. 0. 1.
   you are NOT using Gamma-specific algorithms. phcg.x expects a file
   produced by pw.x with Gamma-specific algorithms.

2) make a phonon calculation for the Gamma point.
   (input=sih4.nm.in, output=sih4.nm.out)
   Note that the calculation is not intended to be a good one,
   but just a test one! Rotation modes have negative frequencies.
   This is a consequence of the supercell approach. Translational
   modes have zero frequency because the translational Acoustic Sum 
   Rule (ASR) is imposed by construction in the calculation
   (option asr=.true.)

3) calculate the IR cross section (input=sih4.dyn.in, output=sih4.dyn.out).
   By applying the appropriate ASR for molecules (option asr='zero-dim')
   the rotational modes are forced to have zero frequency as well.