units of third of force constants

[rfhari]

Hi,

I'm working on calculating the second- and third-order force constants (FC2 and FC3) for AlN and GaN using ALAMODE with VASP. I need these force constants in ShengBTE format so I can use AlmaBTE to calculate the thermal conductivity of AlxGa(1-x)N. I understand that I can output FC2 in the .hessian file and rewrite it into ShengBTE format, and I can use the "FC3_SHENGBTE" tag to obtain FC3 in ShengBTE format as well.

However, I'm unsure about the units of FC3. Are the values in PREFIX.FORCE_CONSTANT_3RD given in Ryd/Bohr^3, or are they converted to eV/A^3? For FC2, this seems clearer as the header of the .hessian file specifies Ryd/Boh^2, but for FC3, there is no explicit mention of units in the PREFIX.FORCE_CONSTANT_3RD file. It would really great if you can clarify this!

Thanks!

[ttadano]

The unit of FC3 saved in PREFIX.FORCE_CONSTANT_3RD is eV/Å^3.