Dear Maximilian,
welcome to the VASP Forum!! Indeed the D3-BJ parameters for R2SCAN are not hardcoded in the VASP source. Since you were using the older VASP 6.4.1 I also checked the current release but also there the parameters are not loaded automatically.
Fortunately, I was able to find the parameters in the source code of the external library simple-dftd3 here: https://github.com/dftd3/simple-dftd3/b ... #L280-L281:
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VDW_A1 = 0.49484001
VDW_A2 = 5.73083694
VDW_S8 = 0.78981345
The link above also hints at a publication where you can find these parameters: https://doi.org/10.1063/5.0041008 (note that the title suggests R2SCAN + DFT-D4 but in TABLE I you can find also values for DFT-D3).
There is one small caveat though using these numbers directly with your existing VASP 6.4.1 installation: the implementation which is selected via IVDW = 12 does not compute the three-body Axilrod–Teller–Muto term (ATM). However, it seems this was included when the parameters were fitted (see the publication). So, if you provide the numbers to VASP 6.4.1 with IVDW = 12 one could argue that there is an inconsistency in the calculations. On the other hand, typically the contributions from the ATM term are rather small and there may be no relevant change in the results.
If you want to include the ATM term we can offer this option: starting with VASP 6.6.0 there is an interface to the already mentioned simple-dftd3 library which is described here: https://vasp.at/wiki/Simple-DFT-D3. You will have to compile and link VASP together with this library and use instead of IVDW = 12 these tags in your INCAR file:
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IVDW = 15 # selects simple-dftd3 library as source for DFT-D3
SDFTD3_XC = r2scan # passes functional to simple-dftd3 library
SDFTD3_DAMPING = rational # set damping function
VDW_S9 = 1 # activates ATM term
The values for A1, A2 and S8 are then automatically retrieved from the simple-dftd3 library.
Hope this helps!
All the best,
Andreas Singraber
