[CP2K-user] [CP2K:11870] Is it necessary to perform frequency calculations for the optimized geometry in cp2k?

[Mohammad Shakiba]

Dear Kee Chon Wee,

Thank you so much for your help.

On Tuesday, June 11, 2019 at 1:54:09 PM UTC+4:30, Kee Choon Wee wrote:
>
> Hi Shakiba,
>
> The purpose of a frequency calculation is to verify that the stationary 
> point you found is not a saddle point (1 imaginary frequency, i.e a TS) or 
> higher-order saddle point with (>1 imaginary frequency).
> I don't think it has anything to do with getting struck in a local minima, 
> In fact, if you have no imaginary frequency the structure you found is a 
> minima (could be a global/local one).
> You can try a short MD to see if there is a large transition within say a 
> picosecond, this might give you a hint as to whether it is a minima or 
> transition state.
>
>
> On Mon, Jun 10, 2019 at 8:32 PM Mohammad Shakiba <msha... at gmail.com 
> <javascript:>> wrote:
>
>> Hello everyone,
>>
>> When optimizing with CP2K, we set a force limit and a maximum step size 
>> (for example less than 10meV and 0.002Bohr) and when the calculations 
>> reached those limits it is done. Before using CP2K I used Gaussian 09. When 
>> the geometry optimization was fully performed, I performed frequency 
>> calculation in order to check that there is no imaginary frequency so the 
>> optimization is not stuck in the local minima. However, when it came to big 
>> molecules, the rwf file tended to be hundreds of GB, so the calculations 
>> was not possible. Now, my question is:
>>
>>
>> Is it necessary to perform frequency calculations (like phonon 
>> calculations using phonopy) for the optimized geometry in CP2K? I mean the 
>> BFGS algorithm for geometry optimization is guaranteed to not stuck in the 
>> local minima? For example my system contains a quantum dot with many 
>> organic ligands (near 300 atoms) and the optimization is fully performed 
>> but I don't know if it would be acceptable to allege that this is the 
>> optimum geometry!  To sum up: when can we say that this is optimum geometry 
>> for this specific geometry configuration?
>>
>> Thanks in advance. 
>>
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