These measure the mixing of the ground and excited states under the nuclear displacements Q , and therefore K v is termed the vibronic contribution. Together with the K 0 value and the energy gap 2 Δ Δ 0 n = E n − − E 0 between the mixing states, F 0 n are the main parameters of the PJTE (see below). In a series of papers beginning in 1980 (see references in ) it was proved that for any polyatomic system in the high-symmetry configuration