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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.
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