Critical role of quantum dynamical effects in the Raman spectroscopy of liquid water

Abstract

Understanding the Raman spectroscopy at the atomistic level is important for the elucidation of dynamical processes in liquid water. Because the polarizability (or its time derivative) is often a highly nonlinear function of coordinates or/and momenta, we employ the linearized semiclassical initial value representation for quantum dynamical simulations of liquid water (and heavy water) under ambient conditions based on an ab initio based, flexible, polarizable model (the POLI2VS force field). It is shown that quantum dynamical effects play a critical role in reproducing the peaks in the intermediate region between the librational and bending bands, those between the bending and stretching bands, and the double-peak in the stretching band in the experimental isotropic Raman spectrum. In contrast, quantum dynamical effects are important but less decisive in the anisotropic Raman spectrum. By selectively freezing either the intramolecular O-H stretching or H-O-H bending mode, we demonstrate that the peak in the intermediate region (2000-2400 cm -1 ) of the isotropic Raman spectrum arises from the interplay of the stretching and bending motions while a substantial part of the peak in the same intermediate region of the anisotropic Raman spectrum may be attributed to the combined motion of the bending and librational modes.

Inexpensive modeling of quantum dynamics using path integral generalized Langevin equation thermostats

“…4, the position of the stretching band of the reduced depolarized Raman spectrum obtained from revPBE0+D3, computed using the time derivative of the instantaneous polarizability of the system following the procedure of Ref. [55], is in excellent agreement with both experimental result and previous calculations [25]. As in the previous case B3LYP+D3 systematically red shifts the stretching band with respect to the experimental result [51].…”

On the “Matsubara heating” of overtone intensities and Fermi splittings

“…Real-time coherence (which usually gets the blame when path-integral methods do not work) thus plays no major role here (and treatments more sophisticated than harmonic PT would be required to determine whether it plays a minor role). These findings are consistent with earlier studies based on the LSC-IVR method 72 which, like path-integral methods, omits real-time coherence. 42 It is probably unrealistic to expect that practical path-integral methods can be generalised to include Matsubara heating, given that the Matsubara fluctuation modes carry large phases.…”

Which quantum statistics–classical dynamics method is best for water?

“…18 Evaluating the Wigner quasi-probability distribution ρ W requires some level of approximation for any system with more than a few degrees of freedom. One approach that has been applied successfully to liquid water is the local Gaussian approximation (LGA) due to Liu and Miller, 11,13,15,16,18 which is based on the local harmonic approximation (LHA) introduced earlier by Shi and Geva. 9 The LGA (like the LHA) makes a locally harmonic approximation to the offdiagonal matrix elements of e −βĤ , allowing the Fourier transforms in ρ W to be evaluated analytically.…”

“…The oldest such 'quantum statistics-classical dynamics' method goes back to Wigner, but was developed into a practical simulation method much more recently; this method is usually referred to as the linearised semiclassical initial value representation (LSC-IVR). [6][7][8][9][10][11][12] The LSC-IVR method has now been used to calculate a variety of condensed-phase observables, including infrared (IR) [13][14][15] and Raman 16 spectra for liquid water. The drawback of LSC-IVR is that the Newtonian dynamics that it employs does not conserve the quantum Boltzmann distribution, which introduces errors into the time-correlation functions (typically in the form of artificially rapid decorrelation).…”