[QE-users] Questions related to vibrational analysis (for both molecule and adsorbate)--update

[Tone Kokalj]

On 2020-03-16 01:53, Shen, Ziheng wrote:
> Thank you so much Paolo!
> 
> I also have some follow up questions, I would appreciate it if you
> could help me with them!
> 
> 1) May I ask is there any literature that can prove it’s feasible to
> neglect the small frequencies? And is this also a same criteria for a
> slab system (if I calculate frequencies for all modes)?

You do not take the six lowest frequencies (roto-translation) into 
account, whereas for the other low frequencies, there is a suggestion of 
Cramer-Truhlar (dx.doi.org/10.1021/jp205508z) to raise the frequencies 
below 100 cm^-1 to 100 cm^-1 (this corresponds to 1/2 RT at T~300K) as a 
way to correct for the breakdown of the harmonic oscillator. While low 
frequencies contribute close to nothing to ZPE, they contribute 
enormously to vibrational entropy. This correction also improves the 
"numeric stability" of vibrational entropy, because low frequencies are 
"numerically difficult" to get precise. I am using it all the time.


> 2) I saw you also replied to others that “nat_todo” does not make
> any sense, but there were still people claimed that they got some
> useful results. I read from some books (like the one written by
> D.Sholl) saying that it’s possible to calculate only frequencies of
> adsorbates

For the adsorbate-surface, what I do is that I fix no atoms, do a full 
relaxation and then calculate all vibrational modes (hence, no 
nat_todo). It is computationally heavy, but it works!

Best regards, Tone
-- 
Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
(tel: +386-1-477-3523 // fax: +386-1-251-9385)