Vibronic couplings pseudo-Renner-Teller coupling

These overtone REPs have much in common with and thus may be difficult to distinguish from those due to pseudo-Renner-Teller coupling between excited states, discussed in Section IV,C and illustrated in Fig. 12. Conceptually, the present vibronic coupling mechanism is simpler than the pseudo-Renner-Teller mechanism in that it involves only a single excited state. It is restricted, however, to modes that are infrared active, whereas the pseudo-Renner-Teller mechanism applies to all modes. For all of these mechanisms, only one element of the scattering tensor is nonzero at resonance so that Pi = j without dispersion. 

Totally symmetric modes are not subject to symmetry restrictions. Their potentials may contain odd and even terms in Q so that the harmonic-oscillator approximation imposes unwarranted symmetry restrictions. Similarly, the corresponding vibronic coupling operator may contain both odd and even terms so that the distinction between pseudo-Jahn-Teller and pseudo-Renner-Teller coupling disappears. Since the potential energy minimum of a totally symmetric mode is different in different electronic states, the pseudo-Jahn Teller/Renner-Teller limit is quite different from the limiting cases discussed in Section I V,B,C. Finally, the transition moments... 

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