Use of CBT

GROMACS version: 2019.3
GROMACS modification: No

Hi, I’m investigating the use of the Combined Bending-Torsion (CBT) potential for a CG FF, and I have a question on how to implement it.

Both the GROMACS 2019 documentation ( and the Marrink et al. paper (J. Chem. Theory Comput. 2013, 9, 3282-3292) state that there are 6 parameters to this potential: k(phi) and a(n) where n = 0 to 4.

However when I look at the file formats page of the GROMACS documentation ( and I look at the CBT potential arguments requirements, only a(n) are listed, i.e. k(phi) is not.

Is this is a mistake in the documentation, or is k(phi) taken as a set value by GROMACS?