[CP2K:8592] imaginary frequencies of water dimer even with tight convergence parameters

[Ali KACHMAR]

Hi Luis,

BTW, The one water molecule is not also straight forward. But, I can assure
you if you do for one water molecule you will do for 2, 3, 4 molecules.

Don't be surprised! Getting negative frequencies is quite common not only
in cp2k, but also in other DFT codes.

those are the frequencies for H2O
        2.165588
       64.392500
      146.662981
     1613.021758
     3717.933517
     3822.394550

You are not missing anything rather than you have to insist to get
converged geometries.

Best, Ali

On Thu, Jan 26, 2017 at 5:41 PM, Matt W <MattWa... at gmail.com> wrote:

> What could be a harder test case than small water clusters?
>
> Matt
>
> On Thursday, January 26, 2017 at 2:38:56 PM UTC, Luis Ruiz Pestana wrote:
>>
>> Hi Ali,
>>
>> Thanks for the response! I am using 400 Ry because I needed it to be the
>> same cutoff that I used in some other AIMD simulations in the
>> condensed phase. Regardless, I have tried the suggested changes and I still
>> get imaginary frequencies. I am quite surprised that cp2k is having a hard
>> time with such simple small systems, am I missing something?
>>
>> Best,
>> Luis
>>
>> On Wed, Jan 25, 2017 at 9:35 AM, Ali KACHMAR <ali.k... at gmail.com>
>> wrote:
>>
>>> Dear Luis,
>>>
>>> 1. a cutoff of 400 Ry is a bit low for geometry optimization +
>>> vibrational frequency calculations on water systems. Please run your
>>> geometry optimization with tight convergence and a cutoff=550 Ry,
>>> Relcutoff=70 Ry
>>>
>>> 2. Starting from the optimized geometry, please run your vibrational
>>> analysis calculation using a DX=0.01, and add to the GEO_OPT section
>>>
>>>   &GEO_OPT
>>>     TYPE MINIMIZATION
>>>     OPTIMIZER CG
>>>   &END GEO_OPT
>>>
>>>
>>> All the best,
>>> Ali
>>>
>>>
>>> On Tue, Jan 24, 2017 at 6:00 PM, <lar... at lbl.gov> wrote:
>>>
>>>> I am trying to compute the vibrational frequencies of small water
>>>> clusters, and I am having trouble getting rid of imaginary frequencies,
>>>> even for the water dimer, using very tight convergence parameters during
>>>> the geometry optimization.I have also tried to run the simulations with
>>>> periodic boundary conditions, different values of the displacement, etc.,
>>>> but I still get a few imaginary frequencies. I have run out of ideas to try
>>>> so any advice will be greatly appreciated. I have attached here the input
>>>> files of the calculations.
>>>>
>>>> Thanks for your time!
>>>> Luis
>>>>
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>>
>>
>>
>> --
>> Luis A. Ruiz Pestana, PhD
>> Postdoctoral Fellow
>> University of California, Berkeley
>> Lawrence Berkeley National Laboratory
>> Berkeley, CA 94720
>>
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