“…These include the set of sum rules that are derived by using the gauge invariance, the virial theorem, the coordinate scaling of electrons, the adiabatic connection, and so on [1,2,9,10,29,30]. Using such sum rules as constraints, we have recently proposed the vorticity expansion approximation (VEA) for the xc energy functional of the CDFT [31][32][33]. This approximate form is developed to fulfill the gauge invariance and nineteen sum rules.…”