[CP2K-user] [CP2K:12784] Negative frequencies in vibrational analysis

[Lucas Lodeiro]

>From your last message I understand that you want to analyze this atoms and
molecules in the gas phase. If this is correct, the box must be big enough
for the PW basis and to ensure the molecule/atom does not interact with its
own images if PBC is activated. A cubic cell of 15 Angstroms should be
enough, if your system is a big molecule, a big cell may be necessary if
PBC is activated.
You need to do a Geometry Optimization to reach a minimum. In the case of
Atoms, you do not need it, and with atoms you can not found vibrational
modes. With an optimized geometry do the vibrational analysis.

The PBC is applied in the cases when you want to simulate a continuum
system, as solids or liquids systems. If you want to simulate a big
molecule in the gas phase, desactivate PBC.

Regards

El vie., 17 ene. 2020 a las 7:39, sumit agrawal (<sumitag... at gmail.com>)
escribió:

> 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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