Evaluation of the Vibrational Spectra Using Classical MD Simulations

Evaluation of the Vibrational Spectra Using Classical MD Simulations 

The IR spectrum is obtained as the Fourier transform of the autocorrelation function of the classical dipole moment M [90], calculated at every point of the MD trajectory 

The observed intensities also depend on the refractive index, vhich in general is frequency dependent [93], This dependence is unkno vn in most cases and has not been considered. We note, however, that for liquid water the refractive index is virtually constant between 300 and 3500 cm i [94]. The dipole autocorrelation function is calculated classically and quantum corrections [95, 96] are introduced through the factor 2/[l+exp(-ko/2 tkBT)]. Eq. (9.16) for the absorption spectrum has previously been applied in calculations of the far- and mid-IR spectra of liquid water [90, 97[ and crystals [85]. The quantum correction damps the intensities of the low frequency motions and more sophisticated schemes [98] may lead to more severe damping of the low frequencies - as found for liquid water [99]. 

Environmental effects on the structure, the PES of the O -H -O fragment, and the vibrational spectrum of H5O2+. 

The relation between the IR and INS spectra specifically the broad IR bands compared to the relatively narrow INS bands for the O -H -O vibrations [110]. 

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