[CP2K-user] [CP2K:12777] Negative frequencies in vibrational analysis

Travis polla... at gmail.com
Fri Jan 17 18:41:38 UTC 2020


If you're interested in pursuing the PBC route, you'll want to create a box 
like the one attached. Start by downloading a structure for your matrix 
from https://materialsproject.org/. You'll want to ensure you're using a 
structure corresponding to the correct space group. Insert your molecule in 
the box. Perform a GEO_OPT (ion relaxation) then a CELL_OPT (lattice + ion 
relaxation, use fixed angles though since you should expect a cubic 
structure). Then do your vibrational analysis. A minimum energy structure 
will have 0 negative/imaginary frequencies.

Alternatively, you can delete all but the nearest Ar atoms and run in gas 
phase (basically what you're doing now). Run GEO_OPT, then vibrations.


On Friday, January 17, 2020 at 6:39:07 AM UTC-4, sumit agrawal wrote:
> Hi Lucas Lodeiro,
> Thanks for your response. Actually I want to do vibrational analysis in 
> the matrix such as Ar and N2. So basically I want to make a cubic cell of 
> the Ar or N2 and i want to put my molecule of interest inside this cube. So 
> can you tell me what is the exact procedure to do such kind of 
> calculations? Should I have to make a cubic cell first then optimized first 
> and after optimization of this cubic cell I have to put my molecule of 
> interest inside this and again I have to optimize this system? or simply 
> should I make a system of interest and optimize this. 
> And one more inquiry I want that my interest in systems has less than 100 
> atoms so should I have to apply PBC of such kind of systems?
> Thanks, Sumit
> On Thursday, January 16, 2020 at 7:31:13 AM UTC-8, Lucas Lodeiro wrote:
>> Hi Sumit,
>> If a vibrational mode presents a "negative" frequency (really it is an 
>> imaginary frequency, due to the hessian element is the negative one), it is 
>> because your system is not at minimmum stationary point (it could be in a 
>> 1° order saddle point). To fix it you must to do a geometry optimization 
>> first, to reach a stationary point, then a vibrational analysis will have 
>> useful information.
>> It is important to note that a vibrational analysis with Harmonic 
>> approximation does not have sense if you do not found a stationary point on 
>> the Potential Energy Surface.
>> Regards - Lucas Lodeiro
>> El mar., 14 ene. 2020 a las 9:23, sumit agrawal (<sum... at gmail.com>) 
>> escribió:
>>> When I am trying to do to vibrational analysis, there are some modes 
>>> that have negative frequencies. How can I solve this issue? 
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