mirror of https://gitlab.com/QEF/q-e.git
28 lines
1.2 KiB
Plaintext
28 lines
1.2 KiB
Plaintext
|
|
This example shows how to use pw.x and ph.x to calculate the normal modes
|
|
of a molecule (CH4).
|
|
|
|
1) make a self-consistent calculation at Gamma.
|
|
(input=ch4.scf.in, output=ch4.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.
|
|
|
|
2) make a phonon calculation for the Gamma point.
|
|
(input=ch4.nm.in, output=ch4.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=ch4.dyn.in, output=ch4.dyn.out).
|
|
By applying the appropriate ASR for molecules (option asr='zero-dim')
|
|
the rotational modes are forced to have zero frequency as well.
|