High dplumed values and zero exchange probability

[LubaSim]

Dear Plumed Users,

I'm running FEP/HREX simulation using Gromacs-2021.7 patched with plumed-2.8.3. However, I get zero probability exchange between the replicas and also high dplumed values, although the difference between the potential energy values in two adjacent replicas is not high. I would like to calculate manually the dplumed energy in order to check that.
According to the code /data/plumed-2.8.3/patches/gromacs-2021.7.diff/src/gromacs/mdrun/replicaexchange.cpp, dplumed is calculated as:
dplumed=adb * re->beta[a]+bdb * re->beta[b]

Could you please help me to understand how the values "adb" and "bdb" are calculated or where I can find them (I guess these are potential energies).

Thanks in advance,
Luba