Sensitivity of semiclassical vibrational spectroscopy to potential energy surface accuracy: A test on formaldehyde.

A set of permutationally invariant potential energy surfaces for the electronic ground state of formaldehyde is built at several levels of electronic theory and atomic orbital basis sets starting from a database of more than 34000 ab initio energies. The reliability of the fitted surfaces is determined by comparing the calculated harmonic frequencies with the corresponding ab initio values. Semiclassical estimates of the quantum frequencies of vibration are presented, and their dependence on the employed level of theory, type of atomic orbital basis set, and complexity of the fit is investigated. Comparisons to experimental data show that anharmonic frequencies are influenced by the precision of the fit, while accurate frequency values are obtained also with density functional theory. Results and conclusions support the use of ab initio \u201con-the-fly\u201d semiclassical dynamics as a means of spectroscopic investigation when high level analytical potential energy surfaces are not available

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

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 the work of 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 for the physical interpretation of the very challenging proton transfer modes. We find that it is important, for such a floppy molecule, to selectively avoid initially exciting lower energy modes, in order to obtain cleaner spectra. The estimated vibrational energies display a mean absolute error (MAE) of 3c29 cm-1 with respect to available multiconfiguration time-dependent Hartree calculations and MAE 3c14 cm-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

Divide-and-Conquer Semiclassical Dynamics: A Viable Method for Vibrational Spectra Calculations of High Dimensional and Anharmonic Molecular Systems

The prediction of accurate vibrational frequencies is often necessary for the interpretation of experimental outcomes, especially when sources of strong anharmonic effects such as hydrogen bonding are present. Unfortunately, the most relevant stumbling block to fill in the gap between theory and experiment is usually represented by dimensionality problems, when quantum mechanical effects like Zero Point Energy, quantum anharmonicities, and overtones cannot be neglected. In this circumstance quantum applications are generally limited to small and medium sized molecules. One possible alternative is represented by Semiclassical theory, which allows to recover accurate spectral densities by taking advantage of quantities arising from classical mechanics simulations. [1-5] In particular, here we present a method, called Semiclassical \u201cDivide-and-Conquer\u201d, able to reproduce spectra of high-dimensional molecular systems accurately. [6,7] The method is first validated by performing spectra of small and medium sized molecules, and then it is used to calculate the spectra of benzene and a C 60 model, which is made of 174 degrees of freedom. Then, we show results of variously sized-water clusters characterized by strong hydrogen-bonding that red shifts the involved OH stretches. [8] Finally, the method is combined with ab-initio molecular dynamics to abandon the necessity to employ pre-fitted Potential Energy Surfaces, and applied to study supramolecular systems like the protonated glycine dimer and hydrogen-tagged protonated glycine. [9] [1] W. H. Miller, J. Chem. Phys. 1970, 53, 3578; [2] E. J. Heller, J. Chem. Phys. 1981, 75, 2923; M. F. Herman and E. Kluk, Chem. Phys. 1984, 91, 27. [3] K. G. Kay, J. Chem. Phys. 1994, 101, 2250; W. H. Miller, J. Phys. Chem. A 2001, 105, 2942. [4] A. L. Kaledin and W. H. Miller, J. Chem. Phys. 2003, 118, 7174. [5] R. Conte, A. Aspuru-Guzik, and M. Ceotto, J. Phys. Chem. Lett. 2013, 4, 3407. [6] M. Ceotto, G. Di Liberto, and R. Conte, Phys. Rev. Lett. 2017, 119, 010401. [7] G. Di Liberto, R. Conte, and M. Ceotto, J. Chem. Phys. 2018, 148, 014307. [8] G. Di Liberto, R. Conte, and M. Ceotto, J. Chem. Phys. 2018, 148, 104302. [9] F. Gabas, G. Di Liberto, R. Conte, and M. Ceotto In preparation

Semiclassical vibrational spectroscopy : the importance of quantum anharmonicity in supra-molecular systems

Semiclassical (SC) vibrational spectroscopy has been applied successfully to several molecular systems thanks to the possibility to regain quantum effects accurately starting from short-time classical trajectories.[1-5] Larger molecular and supra-molecular systems represent instead an open challenge in the field of semiclassical spectroscopy mainly due to the necessity to work in very high dimensionality. To start off the talk I will present some recent theoretical advances able to extend the range of applicability of SC vibrational spectroscopy to very high-dimensional systems.[6-7] Then, I will move to applications of semiclassical spectroscopy concerning the vibrational features of water clusters and two supra-molecular systems involving glycine.[8-9] These applications will point out the importance of a multi-reference, dynamical approach able to reproduce quantum anharmonicities without employing any ad-hoc scaling factor. [1] M. F. Herman, E. Kluk, Chem. Phys. 1984, 91, 27. [2] A. L. Kaledin, W. H. Miller, J. Chem. Phys. 2003, 118, 7174. [3] M. Ceotto, S. Atahan, G. F. Tantardini, A. Aspuru-Guzik, J. Chem. Phys. 2009, 130, 234113. [4] R. Conte, A. Aspuru-Guzik, M. Ceotto, J. Phys. Chem. Lett. 2013, 4, 3407. [5] F. Gabas, R. Conte, M. Ceotto, J. Chem. Theory Comput. 2017, 13, 2378. [6] M. Ceotto, G. Di Liberto, R. Conte, Phys. Rev. Lett. 2017, 119, 010401. [7] G. Di Liberto, R. Conte, M. Ceotto, J. Chem. Phys. 2018, 148, 014307. [8] G. Di Liberto, R. Conte, M. Ceotto, J. Chem. Phys. 2018, 148, 104302. [9] F. Gabas, G. Di Liberto, R. Conte, M. Ceotto, to be submitted

Semiclassical vibrational spectroscopy with Hessian databases

We report on a new approach to ease the computational overhead of ab initio "on-the-fly" semiclassical dynamics simulations for vibrational spectroscopy. The well known bottleneck of such computations lies in the necessity to estimate the Hessian matrix for propagating the semiclassical pre-exponential factor at each step along the dynamics. The procedure proposed here is based on the creation of a dynamical database of Hessians and associated molecular geometries able to speed up calculations while preserving the accuracy of results at a satisfactory level. This new approach can be interfaced to both analytical potential energy surfaces and on-the-fly dynamics, allowing one to study even large systems previously not achievable. We present results obtained for semiclassical vibrational power spectra of methane, glycine, and N-acetyl-L-phenylalaninyl-L-methionine-amide, a molecule of biological interest made of 46 atoms

Divide-and-Conquer Semiclassical Dynamics: A Viable Route for Spectroscopic Calculations of High Dimensional Molecular Systems

The accurate prediction of vibrational spectra has become a very challenging task for theoretical methods. The most relevant stumbling block is represented by the necessity to employ quantum methods, since very often quantum effects, like zero point energy, quantum anharmonicities, and overtones, are not negligible to gain insights into the physics of a molecular system. Unfortunately, quantum mechanical methods are usually affected by the so-called curse of dimensionality problem, which limits their applicability to small and medium sized molecules. A viable alternative is represented by the Semiclassical theory, which is obtained by stationary-phase approximating to the second order of the Feynman Path-Integral representation of the Quantum time evolution operator, and allows to calculate spectral densities. In particular, the Coherent State Representation was shown to be very valid in molecular applications. However, even in this case the curse of dimensionality occurs and the method runs out of steam when the system dimensionality increases to 25-30 degrees of freedom or more. Here, we present a method, called Divide-and-Conquer, able to overcome this issue, and to reproduce spectra of high-dimensional molecular systems, while retaining the typical semiclassical accuracy (20-30 cm-1). The method is tested on simple molecules. Then, it is used to calculate spectra of a C60 model, which is made by 174 degrees of freedom, and of variously sized-water clusters characterized by strong hydrogen-bonding that red shifts the involved OH stretches. Finally, the method is also combined with ab-initio molecular dynamics to abandon the necessity to employ pre-fitted Potential Energy Surfaces, and applied to study supramolecular systems as the protonated glycine dimer and hydrogen-tagged protonated glycine

Laser-excited Fluorescence And Electron-spin Resonance Of Er3+ In Polycrystalline Alcl3

The green fluorescence transitions among the levels corresponding to the 4S3/2 and 4I15/2 configurations of Er3+ diluted in AlCl3 have been measured using laser excitation. The data allow us to determine the crystalline-field splittings of these levels and, in turn, the spin-Hamiltonian parameters. The electron-paramagnetic-resonance spectrum observed at low temperatures is in good agreement with that expected from these parameters. © 1990 The American Physical Society.42190991

Accurate and efficient pre-exponential factor approximations for the semiclassical initial value representation propagator

The semiclassical (SC) theory[1,2] is a very powerful tool to describe molecular reactivity, electronic transitions and molecular vibrations.[3-9] In semiclassical methods quantum informations are obtained by evolving classical trajectories. This is computationally less intense than grid methods. Unfortunately, when the system is complex, the calculation of the SC Herman-Kluk (HK) prefactor[10] is prohibitive, due to the trajectories' instability.[11] For this reason, it is not possible to employ the basic SC-HK approach for large systems. In the past years, several approximations to the HK prefactor have been proposed[12,13] to solve this issue, but they have never been thoroughly assessed. In this work, first we test some of the most common prefactor approximations on small systems. Then, we put forward a new one starting from the Log-Derivative[13] formulation, and that is potentially suitable for bigger systems. This approximation depends only on the Hessian matrix and does not require the calculation of the monodromy matrix elements, which are often unstable. As a consequence, even chaotic trajectories can be employed for vibrational spectra simulations. The results show that our approximation is very reliable for molecules like H2, H2O, CO2, CH2O, CH4 and CH2D2. Future applications of our new prefactor will concern the evaluation of power spectra of large systems, for which quantum calculations are currently out of reach. References [1] W.H. Miller J. Phys. Chem. A 105, 2942 (2001). [2] W.H. Miller Proc. Natl. Acad. Sci. USA 102, 6660 (2005). [3] M. Ceotto, S. Atahan, S. Shim, G.F. Tantardini, and A. Aspuru-Guzik, Phys. Chem. Chem. Phys. 11, 3861 (2009). [4] M. Ceotto, S. Atahan, G.F Tantardini, A. Aspuru-Guzik., J. Chem. Phys. 130, 234113 (2009). [5] M. Ceotto, D. Dell'Angelo, G.F Tantardini, J. Chem. Phys. 133 (5), 054701 (2010). [6] M. Ceotto, G.F. Tantardini, and A. Aspuru-Guzik, J. Chem. Phys. 135, 214108 (2011). [7] R. Conte, A. Aspuru-Guzik, M. Ceotto J. Phys. Chem. Lett., 4, 3407 (2013). [8] D. Tamascelli, F.S. Dambrosio, R. Conte, M. Ceotto J. Chem Phys., 140, 174109 (2014). [9] M.L. Brewer, J.S. Hulme, and D.E. Manolopoulos J. Chem. Phys. 106, 4832 (1997). [10] M.F. Herman, E. Kluk Chem. Phys. 91, 27 (1984). [11] K.G. Kay J. Chem. Phys. 101, 2250 (1994). [12] V. Guallar, V.S. Batista, and W.H. Miller J. Chem. Phys. 110, 9922 (1999). [13] R. Gelabert, X. Gimenez, M. Thoss, H. Wang, and W.H. Miller J. Phys. Chem. A, 104, 10321 (2000)

A Concerted Investigation For Metal/Semiconductor Nanointerface : Interlayer Charge Transfer At Ag/TiO2

In the field of hybrid materials, suitably designed nanoheterojunctions enhance synergistic functionalities and allow to obtain \u201cbrave new materials\u201d with physicochemical properties that are not simply the addition of the precursors\u2019 ones, but are completely new, different, and sometimes unexpected. For these reasons, the use of them has paved the way toward promising applications in many fields, such as electrocatalysis, photocatalysis, electroanalysis, and environmental chemistry, impacting on the everyday life [1]. However, research on such systems is most often dominated by trial and error procedures, while a deep atomistic understanding of the phenomena inside the junction region driving appropriate design of the final device is missing. Here, a concerted theoretical and electrochemical investigation is proposed to gain insights into the important class of heterojunctions made by metal-semiconductor interfaces. The presented case of study is the silver/anatase hybrid nanocomposite, a very promising material for advanced sensing applications [2]. Considering that in most cases titania semiconductors are useless in electroanalysis and silver is subject to fouling and oxidation/passivation, such broad outcomes were totally unexpected. Specifically, Ag/TiO2 interfase provided the first photorenewable sensor device, pushing the limits in terms of accuracy, sensitivity, detection limits, and photoactivity [3]. Despite the ongoing research, a quantitative and comprehensive understanding of the physics behind this nanocomposite is still missing, thus preventing its full exploitation and the extension of the same paradigm to other systems and devices. In particular, cyclic voltammetry and electrochemical impedance spectroscopy are used in combination with periodic plane-wave DFT calculations, giving comparable qualitative, but also quantitative results. We measure the exceptional electrochemical virtues of the Ag/TiO2 junction in terms of current densities and reproducibility, providing their explanation at the atomic-scale level and demonstrating how and why silver acts as a positive electrode [4]. We theoretically estimate the overall amount of electron transfer toward the semiconductor side of the interface at equilibrium and suitably designed electrochemical experiments strictly agree with the theoretical charge transfer estimates. Moreover, photoelectrochemical measurements and theoretical predictions show the unique permanent charge separation occurring in the device, possible because of the synergy of Ag and TiO2, which exploits in a favorable band alignment, in a smaller electron\u2013hole recombination rate and in a reduced carrier mobility when electrons cross the metal\u2013semiconductor interface. Finally, the hybrid material is proven to be extremely robust against aging, showing complete regeneration, even after one year [4]. [1] A.V. Emeline, V.N. Kuznetsov, V.K. Ryabchuk, N. Serpone, Environ. Sci. Pollut. Res. 19 (2012) 3666\u20133675. [2] G. Soliveri, V. Pifferi, G. Panzarasa, S. Ardizzone, G. Cappelletti, D. Meroni, K. Sparnacci, L. Falciola, Analyst 140 (2015) 1486\u20131494. [3] V. Pifferi, G. Soliveri, G. Panzarasa, G. Cappelletti, D. Meroni, L. Falciola, Anal. Bioanal. Chem. 408 (2016) 7339\u20137349. [4] G. Di Liberto, V. Pifferi, L. Lo Presti, M. Ceotto, and L. Falciola, J. Phys. Chem. Lett. 8 (2017) 5372\u20135377