Typically, with a large number of solvent molecules, the bath orbitals in Eq. ( ) only span a small subspace of solvent orbitals. The remaining subspace consists of core orbitals ( occupied solvent orbitals that are not entangled with chromophore ones) and virtual orbitals (χ d = ∑ ν∈E V v νd ϕ ν ). With the partitioning of the solvent orbitals into bath, core, and virtual orbitals, the density matrix undergoes a unitary transformation,