From Equation , ψ can be thought of as a scalar field while the gradient of such a scalar field is a vector field. Therefore, ψ provides static information and the gradient of ψ provides dynamic information about the system. Despite the success of MOs to explain steady-state systems, inserting a molecule between two electrodes and applying a bias across that molecule is a dynamical system and as such, we need the dynamical information to fully describe the effects we see.