The first and second terms in eqn ( ) are, respectively, the multistate (active space) matrix functionals of the kinetic energy, and Hartree-exchange energy: D(r)] between states A and B. The physical interpretation of the TDF is the dynamic correlation contribution to the electronic coupling between two basis states. Although the functional form of the TDF is not known (similar to the exchange-correlation functional in Kohn-Sham DFT), in special cases such as spin-coupling interactions, it can be determined with the constraint of spin-multiplet degeneracy with the high-spin state determined separately using Kohn-Sham DFT. Obviously, as in Kohn-Sham DFT, E AB xc [D(r)] also includes the corresponding residual kinetic energy not expressed in the orbital term.