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Authors: A. Witt, S. Ivanov and D. Marx
Protonated methane, CH5+, has been investigated intensively using a host of different experimental and theoretical techniques since decades, see Ref. [1] for a review. Its challenge to theory arises from its shallow potential energy surface (PES) which leads to dynamical equivalence of the hydrogen atoms often referred to as "scrambling". It has been shown that this is driven by quantum fluctuation effects of the nuclei [2]. In fact, CH5+ can be considered to be one of the smallest, but still most difficult, representatives of a whole class of "fluxional" molecules. Our goal is to compute real-time dynamical properties, in particular IR spectra, of CH5+ and its H/D isotopologues. The IR spectrum of CH5+ simulated with classical ab initio Molecular Dynamics (AIMD) at 300 K and supplemented by the harmonic quantum correction factor (QCF) is in good agreement with experimental spectra measured at 110 K [3]. However, AIMD simulations at the experimental temperature of 110 K show that the scrambling motion (for all isotopologues) is nearly stopped. Additionally, AIMD simulations of the isotopologues at 300 K yield wrong relative peak intensities for the mixed species due to the neglect of nuclear quantum effects. According to Ramirez [4] one should not improve the QCF that influences relative peak intensities, but instead the quality of the time correlation function. In other words, quantum effects that make the difference between H and D should be taken into account explicitly. Therefore, methods that include quantum effects of nuclei are called for. An elegant way is to use Feynman's formulation of statistical mechanics in terms of path integrals and to perform approximate real-time quantum dynamics. One promising approach is known as the "centroid (path integral) molecular dynamics" (CMD) method [5]. The heart of this method is the centroid density of Feynman paths and it turns out that the corresponding dynamics of centroids represents quasi-classical real-time evolution. The applicability of CMD for computing IR spectra has been systematically investigated by us. In this contribution we present IR spectra of CH5+ and its isotopologues computed by means of the adiabatic ab initio CMD approach [6]. The results of those simulations are compared to spectra obtained using (classical) AIMD and to experimental measured spectra. Indeed, wrong relative peak intensities can be corrected when the different "quantumness" of H and D is taken into account. It turns out that ab initio CMD [6] is able to describe the proper dynamics of CH5+ and its isotopologues, since the resulting CMD spectra are in good agreement with experimental ones. References: [1] P. Kumar P. and D. Marx, Phys. Chem. Chem. Phys., v. 8, p. 573 (2006) [2] D. Marx and M. Parrinello, Nature v. 375, p. 216 (1995) [3] O. Asvany, P. Kumar P., B. Redlich, I. Hegemann, S. Schlemmer and D. Marx, Science, v. 309, p. 1219 (2005) [4] R. Ramirez, T. Lopez-Ciudad, P. Kumar P. and D. Marx, J. Chem. Phys., v. 121, p. 3973 (2004) [5] J. Cao and G.A. Voth, J. Chem. Phys., v. 99, p. 10070 (1993) [6] D. Marx, M.E. Tuckerman and G.J. Martyna, Comput. Phys. Commun., v. 118, p. 166 (1999) |
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