  The problem of soliton-soliton force is revisited. From the exact two
solitons solution of a nonautonomous Gross-Pitaevskii equation, we derive a
generalized formula for the mutual force between two solitons. The force is
given for arbitrary solitons amplitude difference, relative speed, phase, and
separation. The latter allows for the investigation of soliton molecules
formation, dynamics, and stability. We reveal the role of the time-dependent
relative phase between the solitons in binding them in a soliton molecule. We
derive its equilibrium bond length, spring constant, frequency, effective mass,
and binding energy of the molecule. We investigate the molecule's stability
against perturbations such as reflection from surfaces, scattering by barriers,
and collisions with other solitons.
