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Nose-Hoover heat bath mass
Date: 2019/09/12 18:56
Name: Pavel Ondracka   <pavel.ondracka@gmail.com>

Hi, I have a question regarding the "heat bath mass" for the Nose-Hoover thermostat.

I was trying to figure out how to set it properly for my system. I did some reading on the topic and ended with some useful formulas, for example for the effective relaxation time teff^2 = Q/(N*kB*T0), where Q is the "heat bath mass", N is the number of degrees of freedom, kB Boltzmann constant and T0 is the temperature.

The problematic part is that in those examples (as well as in the Nose's original article) the units of the Q are [m^2.kg] (to cite Nose: "Q is a parameter of dimension energy&#183;(time)2 and behaves as a mass for the motion of s), while the openmx manual states that the "heat bath mass" is in atomic mass units, which implies to me that the dimension is just mass. How do I convert from the openmx "heat bath mass" parameter to the Q parameter with m^2.kg in SI units as used for example in the Nose's article?

Best regards
Pavel
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Re: Nose-Hoover heat bath mass ( No.1 )
Date: 2019/09/16 12:49
Name: Naoya Yamaguchi

Hi,

According to a source code "MD_pac.c", OpenMX converts parameters about mass by multiplying it by a constant of one unified atomic mass unit (1 u; 1 Da = 1.660538921x10^(-27) kg) divided by the electron rest mass. And, OpenMX basically uses Hartree atomic units, so I think that you can convert from a value of "heat bath mass" into Q in SI by multiplying a_0^2*(1 u) ~ 4.6499832x10^(-48) m^2kg, where the Bohr radius a_0 is equal to 0.529177249x10^(-10) m.

Regards,
Naoya Yamaguchi
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