2D-Raman-THz spectroscopy: A sensitive test of polarizable water models

Abstract

In a recent paper, the experimental 2D-Raman-THz response of liquid water at ambient conditions has been presented [J. Savolainen, S. Ahmed, and P. Hamm, Proc. Natl. Acad. Sci. U. S. A. 110, 20402 (2013)]. Here, all-atom molecular dynamics simulations are performed with the goal to reproduce the experimental results. To that end, the molecular response functions are calculated in a first step, and are then convoluted with the laser pulses in order to enable a direct comparison with the experimental results. The molecular dynamics simulation are performed with several different water models: TIP4P/2005, SWM4-NDP, and TL4P. As polarizability is essential to describe the 2D-Raman-THz response, the TIP4P/2005 water molecules are amended with either an isotropic or a anisotropic polarizability a posteriori after the molecular dynamics simulation. In contrast, SWM4-NDP and TL4P are intrinsically polarizable, and hence the 2D-Raman-THz response can be calculated in a self-consistent way, using the same force field as during the molecular dynamics simulation. It is found that the 2D-Raman-THz response depends extremely sensitively on details of the water model, and in particular on details of the description of polarizability. Despite the limited time resolution of the experiment, it could easily distinguish between various water models. Albeit not perfect, the overall best agreement with the experimental data is obtained for the TL4P water model. © 2014 AIP Publishing LLC. [http://dx

Notes on simulating two-dimensional Raman and terahertz-Raman signals with a full molecular dynamics simulation approach

“…Finally, we examine the sensitivity of the 2D THz-Raman signals to the choice of the force field 43 by comparing the results obtained using the TIP4P/2005 (Ref. 61) and POLI2VS (Ref.…”

Low frequency 2D Raman-THz spectroscopy of ionic solution: A simulation study

“…In principle, polarizable ion-water models can be used to achieve a more consistent modeling of the spectra based on the MD simulation. However, in the recent paper, 48 testing various polarizable forcefields (SWM4-NDP, SWM4-POINT and TL4P) indicates that description of polarizability rather than the forcefield itself can drastically alter the feature of 2D Raman-THz signal. So there are hardly convergence of polarizable water models for experimentcomparable theoretical simulation.…”

“…Note that in this work, we only simulated the so-called Raman-THz-THz signals. Hamm and Tanimura 46,48 also proposed the inverted time-ordering THz-Raman-THz signal, which expand time domain of 2D Raman-THz signal into second quadrant. It follows the continuity of the overall response function at t 1 axis and contains different Liouville pathways of the Raman-THz pulse.…”

“…Two-dimensional response function obtained by different theoretical water models was also compared. 48 Furthermore, 2D Raman-THz spectra of various liquids 49 were simulated based on the equilibrium-non-equilibrium hybrid molecular dynamics (MD) simulation.…”

Interpretation of the THz-THz-Raman Spectrum of Bromoform

“…However, this information is encoded in complex, threepoint correlation functions that must be disentangled with the aid of theoretical and computational methods. 30,31,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] This problem of interpretation is further complicated by the number of vibrational states that need to be considered. Whereas infrared-active modes are usually in their vibrational ground state at room temperature and typically only involve single excitations upon illumination, THz-active modes can be thermally excited at room temperature, and multiple transitions between the different states are possible.…”