Anharmonic vibrational eigenfunctions and infrared spectra from semiclassical molecular dynamics

Current simulations of ultraviolet-visible absorption lineshapes, and dynamics of condensed phase systems, largely adopt a harmonic description to model vibrations. Often, this involves a model of displaced harmonic oscillators that have the same curvature. Although convenient, for many realistic molecular systems this approximation no longer suffices. We elucidate non-standard harmonic, and anharmonic effects, on linear absorption and dynamics using a stochastic Schrödinger equation approach to account for the environment. Firstly, a harmonic oscillator model with ground and excited potentials that differ in curvature is utilised. Using this model, it is shown that curvature difference gives rise to an additional sub-structure in the vibronic progression of absorption spectra. This effect is explained, and subsequently quantified, via a derived expression for the Franck-Condon coefficients. Subsequently, anharmonic features in dissipative systems are studied, using a Morse potential, and parameters that correspond to the diatomic molecule H 2 for differing displacements and environment interaction. Lastly using a model potential, the population dynamics and absorption spectra for the stiff-stilbene photoswitch is presented and features are explained by a combination of curvature difference and anharmonicity in the form of potential energy barriers on the excited potential.

Section: B Morse Oscillatormentioning

confidence: 84%

Quantum dissipative systems beyond the standard harmonic model: Features of linear absorption and dynamics

Smith

Dijkstra

2019

“…33, the evaluation of the full absorption spectrum should be feasible. 108,112 √ ω lql + i √ w lp l and…”

Section: Discussionmentioning

confidence: 99%

Reduced rovibrational coupling Cartesian dynamics for semiclassical calculations: Application to the spectrum of the Zundel cation

Bertaina

Liberto

Ceotto

2019

We study the vibrational spectrum of the protonated water dimer, by means of a divide-and-conquer semiclassical initial value representation of the quantum propagator, as a first step in the study of larger protonated water clusters. We use the potential energy surface from [Huang et al., J. Chem. Phys. 122, 044308 (2005)]. To tackle such an anharmonic and floppy molecule, we employ fully Cartesian dynamics and carefully reduce the coupling to global rotations in the definition of normal modes. We apply the time-averaging filter and obtain clean power spectra relative to suitable reference states, that highlight the spectral peaks corresponding to the fundamental excitations of the system. Our trajectory-based approach allows us for physical interpretation of the very challenging proton transfer modes. We find that it is important, for such a floppy molecule, to selectively avoid to initially excite lower energy modes, in order to obtain cleaner spectra. The estimated vibrational energies display a mean absolute error (MAE) of ∼ 29cm −1 with respect to available Multi-Configuration time-dependent Hartree calculations and MAE ∼ 14cm −1 when compared to the optically active experimental excitations of the Ne-tagged Zundel cation. The reasonable scaling in the number of trajectories for Monte Carlo convergence is promising for applications to higher dimensional protonated cluster systems.The Zundel cation is the most representative member of the family of protonated water clusters, towards which many computational efforts are being devoted, mainly motivated by a flourishing of experimental results, 11-16 and the request for higher accuracy. [17][18][19][20][21] In this respect, this molecule is a prototypical example that has been tackled by various approaches, given the great biological relevance of the charge transport mechanism in aqueous solutions. [22][23][24][25][26][27] On the experimental side, the vibrational spectrum of the Zundel cation has been investigated by infrared multiphoton photodissociation spectroscopy 28,29 and noble gases predissociation spectroscopy, in particular Argon and Neon. [30][31][32] The theoretical literature about the vibrational spectrum of the Zundel cation is quite vast, since its strong anharmonicity provides the ideal test-bed for theoretical methods. The PES computed at the level of coupled cluster theory and devised in Ref. 33, has been employed by a plethora of methods for vibrational calculations, such as vibrational configuration interaction (VCI), 34 diffusion Monte Carlo, 32,35 classical molecu-

“…|C n,K | 2 ∝Ĩ φ K (E n ). As shown in details in our previous work, [57] the signed coefficients in Eq. (6) can be calculated from survival amplitudes using the following working formula…”

Section: Theorymentioning

confidence: 99%

An effective semiclassical approach to IR spectroscopy

Micciarelli

Gabas

Conte

et al. 2019

Self Cite

We present a novel approach to calculate molecular IR spectra based on semiclassical molecular dynamics. The main advance from a previous semiclassical method [M. Micciarelli, R. Conte, J.Suarez, M. Ceotto J. Chem. Phys. 149, 064115 (2018)] consists in the possibility to avoid state-tostate calculations making applications to systems characterized by sizable densities of vibrational states feasible. Furthermore, this new method accounts not only for positions and intensities of the several absorption bands which make up the IR spectrum, but also for their shapes. We show that accurate semiclassical IR spectra including quantum effects and anharmonicities for both frequencies and intensities can be obtained starting from semiclassical power spectra. The approach is first tested against the water molecule, and then applied to the 10-atom glycine aminoacid. *