colvar distance d

Dear LAMMPS users,
I am performing ABF calculations using colvar package. I would like to define the reaction coordinate d1-d2 = d as mentioned in the colvar user mannual. However, I could not find the appropriate keyword for this definition. Could you please help me: how to define this distance reaction coordinate through colvar package. Thanks in advance.

Thank you

B.Satyanarayana, Ph. D
Molecular modeling lab

Why not just read the manual and highlight distance? All the information is provided there.