Reproducing Deep Tunneling Splittings, Resonances, and Quantum Frequencies in Vibrational Spectra From a Handful of Direct Ab Initio Semiclassical Trajectories

Abstract

A time-dependent semiclassical approach for vibrational spectra calculations is shown to describe deep tunneling splittings, resonances and quantum frequencies in multidimensional multi-well systems, by propagating a very limited number of classical trajectories. The approach is tested on ammonia by evolving eight trajectories on a full-dimensional PES. Quantum effects are reproduced and results are in good agreement with time-independent quantum calculations. All the features are maintained when ab-initio "on-the-fly" dynamics is adopted, thus demonstrating that pre-computation of the PES can be avoided. The approach overcomes the typical scaling issues of quantum mechanical techniques without introducing any simplifications nor reductions of dimensionality of the problem. The proposed methodology is promising for further applications to systems of major complexity. TOC/ABSTRACT GRAPHIC KEYWORDSSemiclassical dynamics; MC-TA-SC-IVR; ab initio on-the-fly molecular dynamics; Quantum vibrational spectra; Ammonia; Deep resonant tunneling.

“Divide and conquer” semiclassical molecular dynamics: A practical method for spectroscopic calculations of high dimensional molecular systems

“…[76,77,92,93] To further reduce the computational effort, the multiple coherent time averaged SCIVR (MC SCIVR) has been introduced. [58][59][60][61][62]94] In this Section we provide a more detailed explanation of the "divide-and-conquer" strategy previously introduced elsewhere. [75] The idea is to calculate the power spectrum I (E) of Eq.…”

“…It allows to perform on-the-fly semiclassical molecular dynamics simulations given a few input trajectories. [58][59][60][61][62][63][64][65] The approach is amenable to ab initio direct molecular dynamics, thus avoiding the effort to construct an accurate analytical potential energy surface which may be quite demanding especially for large systems, [66][67][68][69][70][71][72][73][74] and permits to faithfully reproduce quantum effects like quantum resonances, [60] intra-molecular and long-range dipole splittings, and the quantum resonant ammonia umbrella inversion. [62] Nevertheless, all SCIVR methods run out of steam when straightforwardly applied to problems involving large-sized systems.…”

On-the-Fly ab Initio Semiclassical Calculation of Glycine Vibrational Spectrum

“…57,58,62,65,66 Furthermore, they can be associated with ab initio direct dynamics with excellent results, 60,61,67−70 a key feature when dealing with molecules the size of glycine or larger. In fact, even if a large variety of very precise PES fitting techniques is available (see, for instance, refs (71−83). )…”

“…For all these reasons, a semiclassical approach appears to be a straightforward choice for an investigation of “multiwell” effects, as indeed recently demonstrated by two of the authors in studying the ammonia vibrational spectrum. 66 …”

Graphics processing units accelerated semiclassical initial value representation molecular dynamics

“…Several applications [58][59][60][61][62][63][64][65] have demonstrated that this approximation is quite accurate.…”