Microscopic gruneisen parameter(γi) = - ln (ωi)/ln V. For the lowest mode for ScF3 at R point this was calculated to be negative. But how would someone explain the negative value for γi at R point due to the softening of this mode (this is called Rigid Unit Mode)as the temperature decreases? Since the volume increased as T goes down(negative thermal expansion) this implies a positive γi to me. I know that there are other ways that this could be explained. But specifically looking for an explanation from the given equation. Open for discussion.   Thanks in advance