[CP2K-user] [CP2K:20625] Re: Barostat/Thermostat mass units?

[Michela Benazzi]

* Degrees of freedom = 3*N atoms, and the units on my answer are J*s2 = 
kg*s2. 
Sorry about the typos. Is the number normalized by the Bohr radius^2?
On Tuesday, August 27, 2024 at 9:06:47 AM UTC-4 Michela Benazzi wrote:

> Hello CP2K users,
> I have previously asked a question about what kind of barostat CP2K uses, 
> and Thomas Kuhne linked a paper on MTTK barostat which was very helpful (
> https://aiichironakano.github.io/phys516/Martyna-Integrator-MolPhys96.pdf
> ).
>
> However, I tried to calculate my barostat mass to understand the units and 
> I am still at a loss - currently, my restart BAROSTAT MASS is 
> 3.1175614526535734E+006. According to the CP2K manual, the units could be 
> kg, m_e or amu. I tried to calculate it myself following the MTTK formula 
> and there is no match:
>
> W = (N_f + 9)* (k_b*T)/2 * (t_p/2pi)^2
>
> where t_p is the time constant (50 fs), T = 1200 K and N_f is degrees of 
> freedom (aka number of atoms? 64 in my system).
>
> My result is 6.0124759e-47 J (kg*m2), which is in units of moment of 
> inertia. Even if I convert it to m_e or amu it does not match:
> 6.0124759e-47 J (kg*m2) 
>
> (or 3.61328477e+27 amu*m2)
>
> (or 6.60031109024e-17 m_e*m2)
>
>
> I was wondering if:
>
>
> 1) there are other units I have not considered in CP2K, or if CP2K 
> internally converts to something else.
>
> 2) Degrees of freedom =/= number of atoms?
>
> 3) I am supposed to get rid of the m2 term by dividing by area in some 
> way, although I am not sure how scientifically sound it would be, since we 
> have a 3D unit cell.
>
>
> Thank you very much!
>

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