Vibrational Spectra from Quasiclassical Direct Ab Initio Dynamics

Book · Mon Nov 01 04:00:00 EST 2004

OSTI ID:990607

Dupuis, Michel ^[1]; Aida, Misako ^[2]

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1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2. Hiroshima Univ., Higashi-Hiroshima (Japan)

The theory of vibrational spectral analysis from classical trajectories has been available for some time.. It is only recently that, with the availability of powerful computers, it has become practical to apply the method to the ab initio calculation of molecular spectra. These spectra account for anharmonicities and couplings of all the modes. The modern application of the approach combines the methodology of direct ab initio dynamics to generate classical trajectories of the molecular system and of Fourier decomposition of the fluctuations of the velocity autocorrelation function to obtain the density of vibrational states. Infrared and Raman intensities are obtained from Fourier decompositions of the fluctuations of the autocorrelation function of the dipole moment time derivative and polarizability time second-derivatives respectively5. In this chapter we review the essential equations of the method and illustrate its application to two simple molecular systems, water H₂O and its deuterated analog D₂O, and water dimer (H₂O)₂. The spectra were generated by applying the protocol of quasiclassical direct dynamics with forces obtained on the fly from accurate electron-correlated ab initio levels of theory. In the quasiclassical protocol the initial conditions of atomic velocities (directions and magnitudes) are chosen to be consistent with the level v=0 for all the harmonic normal modes of the molecule. The calculated spectra are found in excellent quantitative agreement with the experimentally observed spectra. A detailed analysis for D₂O shows that this choice of initial conditions (quasiclassical with v=0 for all the modes) offers an accurate account of anharmonicities and mode couplings. The method holds promise for the calculation of very accurate ab initio spectra, including IR and Raman intensities, directly comparable to experimental spectra for increasingly larger molecules.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 990607
• Report Number(s): PNNL-SA--41277; KC0301020
• Country of Publication: United States
• Language: English

Related Subjects

99 GENERAL AND MISCELLANEOUS
AVAILABILITY
COMPUTERS
DIMERS
DIPOLE MOMENTS
FLUCTUATIONS
HARMONICS
POLARIZABILITY
Raman
SPECTRA
TRAJECTORIES
VELOCITY
VIBRATIONAL STATES
WATER
ab initio
anharmonic
auto-correlation function
direct ab initio dynamics
harmonic
infrared
mode coupling.
molecular dynamics
quasiclassical
vibrational spectrum