[CP2K-user] [CP2K:13088] Forces in BOMD calculations

[Vladimir Rybkin]

Dear all!

I general remark: the forces in CP2K are NOT the Hellmann-Feynman ones. 
That stems from the fact that CP2K uses position-dependent basis functions 
(Gaussians). 

Yours,

Vladimir

четверг, 9 апреля 2020 г., 14:39:33 UTC+2 пользователь Dmitrii Nikolaev 
написал:
>
> Thank you very much! 
> My energy is not shifting, and the forces are fluctuating around 20 
> kcal/(mol*A) -- 0.016 Ht/Bohr. Is it ok? 
>
>
> On Thursday, April 9, 2020 at 3:06:17 AM UTC+3, Lucas Lodeiro wrote:
>>
>> Hi, 
>> In the case of BO framework, the forces calculated with Hellmann-Feynman 
>> theorem have some instabilities, due to the theorem requires that 
>> <d\phi|\phi> = 0 = <\phi|d\phi>. In most cases this terms are very small, 
>> but requires a very accurate convergence of energy and the wfc to ensure a 
>> completely negligible value.
>> Then the real forces are equal <dH> + E<d\phi|\phi>.
>> if <d\phi|\phi> is not negligible, you are avoiding a part of the real 
>> forces and the summation  of them must not be 0.
>>
>> This in the theory. The implementation of this, uses no infinite numbers 
>> and some noise are in the numerical evaluation, another thing to no 
>> cancel completely. 
>>
>> The important thing with this issue is if you are using and NVE system, 
>> the total energy (kinetic + potential) must be the conserved quantity, not 
>> in a perfect numeral way, but the oscillations must be small and without a 
>> biased direction, if the average (along the fluctuations) total energy is 
>> incrementing or decreasing you have problems in the forces application, 
>> either in the force calculation (more accurate scf cycles) or a too big 
>> time step integrator.
>>
>> El mié., 8 abr. 2020 a las 13:59, Dmitrii Nikolaev (<
>> dmi... at gmail.com>) escribió:
>>
>>> Dear collegues,
>>>
>>> I am running the Born-Oppenheimer molecular dynamics, and I notice that 
>>> the sum of the forces on all atoms of my molecule is non-zero. However, as 
>>> I understand for the NVE ensemble the total force should be zero? What is 
>>> the reason for this force? 
>>>
>>> Thank you in advance,
>>> Dmitrii
>>>
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>>
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