Huang & Rhys were among the first to discuss internal conversion in the solid state, studying F-centres. Within the Born-Oppenheimer approximation, a concise description of both the F-centre and the atomic lattice shows that transitions can occur between degenerate states . Importantly, Fermi's Golden rule is valid as long as wavefunction orthogonality is maintained. From here, an expression for the transition probability of internal conversion in solid state materials can be cast as :