In this way, σ λ i (r) can push away or bring closer the i-th electron to the reference electron from these initial a i distances. The σ of Eq. 12 already gives accurate atomic energy densities and captures the physics of bond breaking. Furthermore, even though it is built for 3D systems, the same form gives comparable accuracy for reduced dimensionalities . The MRF's P 2,λ ([ρ]; r, u) that gives rise to Eq. 8 is given by: