IR spectral density of weak H-bonds involving indirect damping. Part III: application to situations with multiple Fermi resonances

The IR spectral density (SD) of the high frequency stretching mode of weak H-bonded complexes involving Fermi resonances is studied within the linear response theory from a full quantum mechanical point of view. The anharmonic coupling between the high frequency X-H-vector...Y and the low frequency X-vector-H...Y-vector modes is treated inside the strong anharmonic coupling theory, whereas Fermi resonances are introduced in the spirit of the work of Witkowski and Wojcik [Chem. Phys. 1 (1973) 9]. The relaxation of the fast and bending modes (direct damping) is treated as usual, whereas the indirect damping is incorporated by aid of our results of part I [Chem. Phys. 293 (2003) 9], dealing with the quantum theory of driven damped quantum harmonic oscillator. The IR SD is obtained by Fourier transform of the autocorrelation function of the dipole moment operator of the fast mode