I use the TINKER molecular modeling software package (version 7.1.2) for computational physical chemistry. There is an extra file (extra1.f) included for the user to edit and incorporate their own unique forces/energies to the dynamics of the molecules. The energy that I want to add contains the Boltzmann constant, but I am having trouble determining if it is correct to use the units of calorie (kcal/mole/K) or joule (kJ/mole/K). The source code seems to use both units in different ways. The default Boltzmann parameter in the user guide and source code is 0.831446215 (kB) in g*Ang2/ps2/mole/K, but the energy totals are all listed in kcal/mol K. Which unit is correct to use in the extra1 file?