Units in "Compute Viscosity"

LAMMPS LAMMPS General Discussion
1

Dear community,

I am trying to compute the viscosity of a liquid using “compute viscosity” together with “fix accelerate/cos”, and I don’t fully understand the units. I am using “units real”. The relevant equations are found on

https://docs.lammps.org/fix_accelerate_cos.html

I refer to the source code example from the “compute viscosity” page:

compute  cos all viscosity/cos
variable V equal c_cos[7]
variable A equal 0.02E-5
variable density equal density
variable lz equal lz
variable reciprocalViscosity equal v_V/${A}/v_density*39.4784/v_lz/v_lz*100

If I see it correctly, the acceleration A is in units of kCal Angstrom^-1 g^-1, the velocity V in units of Angstrom fs^-1, the density in g cm^-3, and Lz in Angstrom. The 39.4784 is the 4pi^2, that’s clear. But how does the factor 100 appear? When I try to obtain units of (Pa*s)^-1 for the reciprocal viscosity, I obtain an additional factor of 238.85 … The “kCal” from the acceleration does not shorten out as far as I see, so the “4.18” should appear somewhere.

Could somebody help me to understand what the resulting output unit of the above code example from the LAMMPS manual is?

Best regards,
Martin

2

Your best shot is to contact the author of that code. Contact info should be in the source code.

3

Thanks for this suggestion. I contacted the author of the code (Zheng Gong), and he immediately replied. There is an inconsistency in the LAMMPS documentation of “fix accelerate/cos”. There it is written that the acceleration is specified in units of force/mass, but it is actually specified in position/time^2. With this new information, everything seems to be consistent now, and the code example from above yields the reciprocal viscosity in (Pa*s)^-1 as expected.

Zheng wrote that he is going to update the LAMMPS documentation.

Best regards,
Martin

2 Likes
4

Thanks for reporting back. This is highly welcome as it avoids frustration in people that search the web for similar issues and see an inquiry without the resolution.