potfit does not need a particular system of units. It can work with any system. The units used by potfit are implicitly defined in the configuration and potential files, and possibly by certain parameters specifying the potential.

The reference configuration file contains atom positions, which implicitly defines a length unit. The interaction potential implicitly makes use of a length unit and an energy unit. Of course, this length unit must be the same as for the configuration file. The two units of length, and energy determine other units - like force and pressure - which are derived from them. The «temperature» in simulated annealing has the same dimension as the sum of squares - none at all.

If the length unit is 1 Å = 1e-10 m, and the energy unit is 1 eV = 1.602e-19 J (which corresponds to 11594 K), then the unit of pressure is 160.2 GPa = 1602 kbar, and the unit of force is 1 eV/Å = 1.602e-9 N = 1.602 nN