P. Blaise et O. Henri-rousseau, Linear response theory and IR spectral density of direct damped weak H-bonds: validity of adiabatic approximation, CHEM PHYS, 243(1-2), 1999, pp. 229-248
The adiabatic approximation (AA) involved in the quantum theory of the IR l
ineshapes (nu(X-H) stretching mode) of weak H-bonds, is revisited. This is
performed directly by looking at the spectral density obtained by Fourier t
ransform of the direct damped autocorrelation function of the transition di
pole moment. The spectral densities computed according to the AA are compar
ed to those obtained directly through the diagonalization of the representa
tive matrix of the Hamiltonian involving the anharmonic coupling between th
e high- and low-frequency modes. At 300 K, when the anharmonic parameter re
mains small (alpha degrees less than or equal to 1), the AA lineshape reduc
es to the exact one, whereas beyond (alpha degrees greater than or equal to
1), the AA is less and less satisfied since its spectral density differs p
rogressively from the exact. Introduction in the AA of effective angular fr
equency and effective anharmonic parameter, allows us for alpha degrees app
roximate to 1.5 to fit sensitively the exact curve except for the spectrum
tails. Connection between the two approaches is established through effecti
ve Hamiltonians (operators and matrices) by means of the wave operator. (C)
1999 Elsevier Science B.V. All rights reserved.