where σ and σ ′ are indexes of AO basis functions on either the chromophore or solvent molecules, C σ i and C σ a are coefficients of canonical occupied and virtual SCF orbitals of the chromophoresolvent complex, respectively, namely eigenvectors of the groundstate Fock matrix in Eq. ( ). When n is equal to 0, Eq. ( ) produces the normal ground-state density matrix, P +0 = P 0 .