TK. Theory and Computation
Tuesday, 2020-06-23, 01:45 PM
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TK01 |
Contributed Talk |
15 min |
01:45 PM - 02:00 PM |
P4561: AN EDGE-SPECIFIC
SCHEME FOR EQUATION-OF-MOTION COUPLED-CLUSTER CALCULATIONS OF X-RAY ABSORPTION SPECTRA |
XUECHEN ZHENG, LAN CHENG, Department of Chemistry, Johns Hopkins University, Baltimore, MD, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK01 |
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We present an edge-specific scheme for calculating near edge x-ray absorption fine structure (NEXAFS) spectra using core-valence separated equation-of-motion coupled-cluster (CVS-EOM-CC) theory. [1,2] Standard correlation-consistent basis set for the atom where targeted core excitation takes place is systematically augmented with diffuse s-, p-, and d-type functions to accurately describe Rydberg-type core excitations with diffuse character.
It is also shown that triple excitations in CVS-EOM-CC methods not only are important for obtaining accurate absolute values of core excitation energies, but also make significant contributions to relative shifts between local and Rydberg core excitations. Quadruples excitations are shown to be relevant when aiming at high-accuracy calculations of absolute values.
Reference
[1] L. S. Cederbaum, W. Domcke, J. Schirmer, and W. von Niessen, Phys. Scripta 21, 481 (1980).
[2] S. Coriani, and H. Koch, J. Chem. Phys. 143, 181103 (2015).
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TK02 |
Contributed Talk |
15 min |
02:03 PM - 02:18 PM |
P4589: AB INITIO CALCULATIONS OF VIBRATIONAL LEVELS AND FRANCK-CONDON FACTORS FOR LASER-COOLABLE MOLECULES |
CHAOQUN ZHANG, LAN CHENG, Department of Chemistry, Johns Hopkins University, Baltimore, MD, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK02 |
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Vibrational energy levels and Franck-Condon factors (FCFs) of laser-coolable molecules has attracted more and more attention because of their promising applications in quantum information science and search for fundamental physics. This presentation focuses on the calculations of vibrational energy levels and FCFs for SrOH and YbOH molecules, which play significant roles in the research of laser cooling. We calculated potential energy surfaces (PESs) for low-lying electronic states of these molecules using scalar relativistic equation-of-motion coupled-cluster (EOM-CC) methods[1]. A standard discrete variable representation (DVR) method[2,3] was then adopted for computations of vibrational energy levels, wavefunctions, and FCFs. The accuracy of the calculations was critically analyzed by comparing computational results with experiments[4,5].
Reference:
[1] J.F. Stanton and R.J. Bartlett, J. Chem. Phys. 98, 7029 (1993).
[2] D.T. Colbert and W.H. Miller, J. Chem. Phys. 96, 1982 (1992).
[3] R.G. Littlejohn, M. Cargo, T. Carrington, K.A. Mitchell, and B. Poirier, J. Chem. Phys. 116, 8691 (2002).
[4] D.T. Nguyen, T.C. Steimle, I. Kozyryev, M. Huang, and A.B. McCoy, J. Mol. Spectrosc. 347, 7 (2018).
[5] E.T. Mengesha, A.T. Le, T.C. Steimle, L. Cheng, C. Zhang, B.L. Augenbraun, Z. Lasner, and J. Doyle, arXiv preprint arXiv:2002.05849 (2020).
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TK03 |
Contributed Talk |
15 min |
02:21 PM - 02:36 PM |
P4591: DIFFUSION MONTE CARLO USING MACHINE LEARNING POTENTIAL ENERGY SURFACES |
MARK A. BOYER, RYAN J. DIRISIO, JACOB M FINNEY, ANNE B McCOY, Department of Chemistry, University of Washington, Seattle, WA, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK03 |
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Diffusion Monte Carlo (DMC) is a technique for obtaining the ground-state solution to the vibrational time-independent Schrödinger equation based on a stochastic sampling of an electronic potential energy surface (PES). To obtain accurate results, one needs a globally-accurate PES. Ideally, the electronic energies used in DMC are calculated at the CCSD(T) level of theory. Recently, neural networks have shown promise to provide accurate post-Hartree corrections to the electronic energy at the cost of a standard Hartree-Fock calculation Welborn, M. , Cheng, L., Miller, T. M. JCTC. 2018 14 (9), 4772-4779Cheng, L., Kovachki, N. B., Welborn, M., Miller, T. M. JCTC. 2019 15 (12), 6668-6677. These neural network surfaces, however, have only been validated using a small sampling of the available configurations of the system. DMC, by design, samples the entire region of the PES that is sampled by the vibrational ground state wave function.
Despite the speed of the neural network potential relative to a calculation at a high level of theory such as CCSD(T), the necessity of the evaluation of the Hartree-Fock energy for every one of the thousands of DMC configurations over tens of thousands of time-steps makes this computationally challenging. To this end, we have developed a DMC algorithm to take advantage of massively parallel high performance computing environments. In addition, to further alleviate the computational cost, we have incorporated importance sampling into the algorithm to reduce the overall number of configurations required to adequately sample the PES. Combined, these two optimizations push the calculation back into the realm of computational feasibility. We have benchmarked the calculation on the water monomer and the CH5+ molecular ion. We plan to extend this work to larger systems, such as large water clusters or a simple Criegee intermediate, where running the DMC with traditional electronic structure would be intractable.
Footnotes:
Welborn, M. , Cheng, L., Miller, T. M. JCTC. 2018 14 (9), 4772-4779
Footnotes:
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TK04 |
Contributed Talk |
15 min |
02:39 PM - 02:54 PM |
P4607: USING A ROBUST ∆SCF METHODOLOGY TO SIMULATE VIBRATIONAL SPECTROSCOPY OF EXCITED STATES |
ALI ABOU TAKA, HECTOR CORZO, AURORA PRIBRAM-JONES, HRANT P HRATCHIAN, Chemistry and Chemical Biology, University of California Merced, Merced, CA, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK04 |
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The use of ∆SCF methods has a rich history in the computational study of electronic excited states. However, such calculations often suffer from a number of challenges including convergence difficulties and variational collapse. Addressing both of these concerns, we have recently developed a projection-operator based approach that significantly improves the robustness of SCF optimization. In this talk, we will describe this new SCF convergence method and demonstrate its ability for studying the vibrational spectroscopy of excited states. Our initial results show that our projection-based scheme provides reliable means for efficiently explore the structure and properties of molecules with electronic structures corresponding to excited states with the efficiency of ground state quantum chemistry models. Results using this method on a set of molecular systems will be compared with results obtained using the more expensive (linear response) time-dependent density functional theory, equation-of-motion coupled cluster, and configuration interaction methods.
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TK05 |
Contributed Talk |
15 min |
02:57 PM - 03:12 PM |
P4629: INSIGHTS ON THE ELECTRONIC AND MOLECULAR STRUCTURES OF LANTHANIDE-BASED CLUSTERS |
HASSAN HARB, HRANT P HRATCHIAN, Chemistry and Chemical Biology, University of California Merced, Merced, CA, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK05 |
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Lanthanide-based clusters provide useful insight to the electronic structure of bulk materials with unique magnetic, electronic, and optical properties. Inspired by photoelectron spectra reported by experimental collaborators and others, we have used density functional theory calculations to explore the molecular and electronic structures of a family of small lanthanide-based clusters. In this talk, we will present our recent results showing intriguing and unique trends in the structure and bonding of these clusters. In addition, the natural ionization orbital analysis was used to determine the nature of electron detachment in photoelectron spectra of these species and to investigate resulting electron rearrangement upon ionization. Such analysis allows us to differentiate between one-electron detachments and shake-up/shake-down transitions.
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TK06 |
Contributed Talk |
15 min |
03:15 PM - 03:30 PM |
P4642: GAUSSIAN PROCESSES FOR SPECTROSCOPICALLY ACCURATE POTENTIAL ENERGY SURFACES OF BENZENE DIMER |
ZACHARY DYOTT, EDWIN SIBERT, Department of Chemistry, University of Wisconsin-Madison, Madison, WI, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK06 |
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Gaussian process regression (GPR) is a robust method for fitting functions due to the flexible ways in which covariance between calculated ab initio single points can be described and the relatively low amount of these training points required to achieve convergence. Increasing the complexity of the covariance (kernel) functions will systematically improve the fidelity of any given fit for functions of up to several dozen variables before the GPR becomes too computationally expensive to perform. This talk will focus on the development of a GPR algorithm to describe a 5-dimensional potential energy surface for the benzene dimer, and subsequent applications of the learned surface to spectroscopic calculations. The high symmetry of benzene allows us to decrease the number of training points even further than previously required while still maintaining faithful representation of the underlying PES. Through systematic engineering of a sufficient kernel function and both stochastic and strategic selection of training points in the conformational space of the benzene dimer, we will have access to a complete map of the necessary molecular parameters to predict accurate IR, Raman, and VSFG spectra for benzene from the gas to the condensed phase.
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TK07 |
Contributed Talk |
15 min |
03:33 PM - 03:48 PM |
P4673: TERT-BUTYL PEROXY RADICAL:GROUND AND FIRST EXCITED STATE ENERGETICS AND FUNDAMENTAL FREQUENCIES |
RONALD M. BERCAW, PETER R. FRANKE, KEVIN B. MOORE III, HENRY F. SCHAEFER III., GARY E. DOUBERLY, Department of Chemistry, University of Georgia, Athens, GA, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK07 |
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Alkylperoxy radicals (RO2 .) are key intermediates in combustion and atmospheric oxidation processes. As such, reliable detection and monitoring of these radicals can provide a wealth of information about the underlying chemistry. The tert-butyl peroxy radical is the archetypal tertiary peroxy radical, yet its vibrational spectroscopy is largely unexplored. To aid in future experimental investigations, we have performed high-level theoretical studies of the fundamental vibrational frequencies of the ground- and first excited states. A conformer search on both electronic surfaces reveals single minimum-energy structures. We predict an <̃span class="roman">A2 A′← <̃span class="roman">X 2 A" adiabatic excitation energy of 7738 cm−1via focal point analysis, approximating the CCSDT(Q)/CBS level of theory. This excitation energy agrees to within 17 cm−1of the most accurate experimental measurement. We compute CCSD(T) fundamental vibrational frequencies via second-order vibrational perturbation theory (VPT2), using a hybrid force field in which the quadratic (cubic/quartic) force constants are evaluated with the ANO1 (ANO0) basis set. Anharmonic resonance polyads are treated with the VPT2 + K effective Hamiltonian approach. Among the predicted fundamental frequencies, the ground state \textO−\textO stretch, excited state \textO−\textO stretch, and excited state \textC−\textO−\textO bend fundamentals are predicted at 1138, 959, and 490 cm−1, respectively. Basis set sensitivity is found to be particularly great for the \textO−\textO stretches, similar to what has already been noted in smaller, unbranched peroxy radicals. Exempting these \textO−\textO stretches, agreement with the available experimental fundamentals is generally good (± 10 cm−1).
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TK08 |
Contributed Talk |
15 min |
03:51 PM - 04:06 PM |
P4678: AN ALGEBRAIC DVR APPROACH TO DESCRIBE THE STARK EFFECT |
MARISOL BERMÚDEZ MONTAÑA, Chemistry Faculty, National Autonomous National of Mexico, Mexico City, Federal District, Mexico; MARISOL RODRÍGUEZ ARCOS, RENATO LEMUS, Estructura de la Materia, Instituto de ciencias Nucleares, Mexico City, Mexico; ORGAZ BAQUE E, Chemistry Faculty, National Autonomous National of Mexico, Mexico City, Federal District, Mexico; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK08 |
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An algebraic approach based on the U(4) algebra is proposed to describe 3D systems for effective potentials. Our approach is based on the addition of a scalar boson to the 3D harmonic oscillator space keeping the constraint of a total number of bosons N constant, in similar form as in the vibron model. However instead of dealing directly with the dynamical symmetries we proceed to identify the coordinates and momenta in the algebraic space. This allows the kets associated with the different subgroup chains to be linked to energy, coordinate and momentum representations. This identification provides powerful tools to obtain the matrix representation of 3D Hamiltonians in a simple form through the use of the transformation brackets connecting the different bases. The exact energy and wave functions are obtained in the large N limit. Because the relevance of the Coulomb interaction we consider as an example of our approach the analysis of the non relativistic Hydrogen atom as a first step to establish the ground to deal with multi-electron molecular systems.
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TK09 |
Contributed Talk |
15 min |
04:09 PM - 04:24 PM |
P4683: SIMULATION OF THE ABSORPTION SPECTRUM OF CHLORINE PEROXIDE (ClOOCl) |
MEGAN R BENTLEY, Chemistry, University of Florida, Gainesville, FL, USA; JOHN F. STANTON, Quantum Theory Project, University of Florida, Gainesville, FL, USA; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK09 |
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Chlorine oxides, namely chlorine monoxide (ClO) and its head-to-tail dimer (ClOOCl), are thought to be important participants in the catalytic destruction of ozone in the Antarctic polar vortex. Chlorine peroxide photolysis is the crucial step in the ozone-depleting mechanism, requiring accurate measurements of absorption cross sections to estimate the amount of ozone destroyed by this process in the polar stratosphere. However, there are large inconsistencies in previous experimental determinations of absorption cross sections in the longer wavelength tail region, where chlorine peroxide photolysis is atmospherically relevant. We have used an admittedly primitive model to construct a simulated absorption spectrum for dissociative excited states, as is the case for chlorine peroxide. The simulated spectrum uses oscillator strengths and excitation energies generated from equation-of-motion coupled-cluster (EOM-CC) techniques that include effects of triple excitations and agrees well with experimental spectra. This method shows promise in elucidating semi-quantitative features of the absorption cross sections in the longer wavelength tail region without encountering common experimental complications.
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TK10 |
Contributed Talk |
15 min |
04:27 PM - 04:42 PM |
P4272: FLUORESCENCE DYNAMICS OF EXCITED STATE PROTON TRANSFER IN SALICYLIC ACID: REVISITED |
HIRDYESH MISHRA, Department of Physics, Banaras Hindu University, Varanasi, Uttar Pradesh, India; |
IDEALS Archive (Abstract PDF) |
DOI: https://dx.doi.org/10.15278/isms.2020.TK10 |
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Excited state intramolecular proton transfer (ESIPT) reaction in salicylic acid (SA) and its derivatives has been the subject of intense investigations. Due to its importance in photo-chemical and photo-biological reactions, ESPT reaction has been reviewed by many workers in number of molecular systems. These compounds are promising in the development of proton transfer lasers, photostabilisers and information storage devices at the molecular level. Like other carboxylic acids, SA also exists as a cyclic hydrogen bonded dimmer in the solid state, in non-polar solvents and in the gas phase at high concentrations. SA dimer is the smallest aromatic system in which both intra and inter molecular hydrogen bonding exist and thus constitute an ideal model to study both inter and intra molecular proton transfer in a single system. This system is found to be complicated due to the presence of two acidic protons, one on the carboxylic group and other on the phenolic group in each monomeric unit in dimmer, which are partially exchanged. The emission spectra of two rotamers and their dimers unfortunately overlap. As a results, there is no reliable studies on the excited electronic states of SA in the condensed phase. SA shows dual emission (UV and blue) in the non-polar as well as in the crystalline state. In the present work, photo physics and photochemistry of SA in crystalline (solid) state have been reinvestigated by both experimental and computational spectroscopic computational calculations technique.
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