Vibration large amplitude vibrational response

The treatment of the vibrational NLO properties in the previous sections employed either the Bishop-Kirtman perturbational theory (BKPT) or the finite field-nuclear relaxation (FF-NR) approach. These approaches may fail for molecules containing large amplitude anharmonic motions, as indeed was suspected to happen in Li C6o-In such cases a more recently proposed variational method, based on analytical response theory [78, 79], would in principle be applicable, but is computationally extremely expensive, as it requires an accurate numerical description of the potential energy surface (PES), at least if the anharmonicity is so large that a power series expansion of the PES is inadequate [80]. 

The plot of these functions, called the frequency response of the system, presents one of the most important features in forced vibration. In a lightly damped system, when the forcing frequency is close to the natural frequency (r 1), the amplitude of the vibration can get extremely large. This phenomenon is called resonance (consequently, the natural... 

---