Full-Dimensional Ammonia Vibrational Spectrum from a Handful of Classical Trajectories

The accurate description of quantum properties in real molecules is a major computational task, due to the growing efforts required as the number of degrees of freedom of the system increases. Different semiclassical methods have been adopted in the attempt to gather quantum properties from computationally cheap classical-trajectories simulations. However, issues related to possible chaotic behavior and classical integrator instability at long simulation times, together with large number of classical trajectories required often to limit the success of basic semiclassical approaches to model systems. In the present work, we demonstrate that the multidimensional double well full vibrational spectrum, including tunnel splitting, can be described quite accurately by generating only 8 classical trajectories if Multi Coherent States Time Averaging SemiClassical Initial Value Representation (MC-TA-SCIVR) is used.[1-4] The first promising results have been obtained on a 1D double-well potential aimed at describing the umbrella motion in ammonia;[5] then, the full-dimensional vibrational spectrum of ammonia is presented, computed on both a high-level fitted PES [6,7] and ab-initio on-the-fly simulations. For the first time in classical-trajectories based methods, both tunnel splitting amplitudes and quantum vibrational frequencies are simultaneously reported.[8,9] Moreover, the exiguous number of trajectories needed makes the present approach reliable for ab-initio on-the-fly approaches, thus avoiding the request for multidimensional PES calculations.[9] References [1] M. Ceotto, S. Atahan, G.F. Tantardini and A. Aspuru-Guzik, Multiple coherent states for first-principles semiclassical initial value representation molecular dynamics, J. Chem. Phys. 130, 234113 (2009). [2] M. Ceotto, S. Atahan, S. Shim, G.F. Tantardini and A. Aspuru-Guzik, First-principles initial value represenation molecular dynamics, PCCP 11, 3861 (2009). [3] M. Ceotto, G.F. Tantardini and A. Aspuru-Guzik, Fighting the curse of dimensionality in first-principles semiclassical calculations: non-local reference states for large number of dimensions, J. Chem. Phys. 135, 214108 (2011). [4] A.L. Kaledin and W.H. Miller, Time averaging the semiclassical initial value representation for the calculation of vibrational energy levels, J. Chem. Phys. 118, 7174 (2003). [5] C.-K. Lin, H.-C. Chang and S.H. Lin, Symmetric Double-Well Potential Model and Its Application to Vibronic Spectra:Studies of InversionModes of Ammonia and Nitrogen-Vacancy Defect Centers in Diamond, J. Phys. Chem. A 111, 9347 (2007). [6] J.M.L. Martin and T.J. Lee, An Accurate ab Initio Quartic Force Field for Formaldehyde and Its Isotopomers, J. Mol. Spect. 160, 105 (1993). [7] N.C. Handy, S. Carter and S.M. Colwell, The vibrational levels of ammonia, Mol. Phys. 96, 477 (1999). [8] A.L. Kaledin and W.H. Miller, TA-SCIVR for vibrational levels. Application to H2CO, NH3, CH4, CH2D2, J. Chem. Phys. 119, 3078 (2003). [9] R. Conte, A. Aspuru-Guzik and M. Ceotto, Full-dimensional ammonia vibrational spectrum from a handful of classical trajectories, in progress.