[CP2K-user] [CP2K:20626] Barostat/Thermostat mass units?

[Krack Matthias]

Hi

CP2K handles all quantities internally in atomic units [a.u.]. You can use the conversion factors printed in the top of the CP2K output for the conversion:


 [a.u.] -> [fs]                                             2.41888432650478E-02

 [a.u.] -> [K]                                              3.15774647902944E+05

The formula for the barostat mass is

M(barostat) = (N(dof) + d)*T*tau^2  (with tau = 1/omega)

d = 3 (dimensional system)
N(dof) = 3*N(atom) – 3 = 3*64 - 3
T = 1200 K / 3.15774647902944E+05 [K/a.u.]
tau = 50 fs / 2.41888432650478E-02 [fs/a.u.]

M(barostat) = (3*64 – 3 + 3)*(1200.0/3.15774647902944E+05)*(50.0/2.41888432650478E-02)**2 = 3117561.45265 a.u.

With periodic boundary conditions (PBC), the degrees of freedom (dof) have to be reduced by 3 (translational dof).
You can ignore k (Boltzmann) when using atomic units.

HTH

Matthias

From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Michela Benazzi <bnzmichela at gmail.com>
Date: Tuesday, 27 August 2024 at 15:09
To: cp2k <cp2k at googlegroups.com>
Subject: [CP2K:20624] Barostat/Thermostat mass units?
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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