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TYC Discussion Meeting: Quantum Chemistry Techniques

Ali Alavi

University of Cambridge & Max Planck Institute for Solid State Research

Mariana Rossi

Fritz-Haber Institut of the Max Planck Society

Sotiris Xantheas

PNNL

Friday 12th April 2019
Time: 3pm
Venue: G21 Ramsay LT followed by a reception in the Nyholm
Contact: Karen Stoneham
Tel: 0207 6797306

Ali Alavi

Non-unitary quantum chemistry: {\em The} route to handling all aspects of the electron correlation problem?

Non-Unitary similarity transformations of the quantum chemical Hamiltonian generally result in non-Hermitian problems, which have generally been shunned in quantum chemistry because of difficulties in applying variational methods to such problems. Projective techniques such as the Full CI quantum Monte Carlo method,  however, can be applied quite straightforwardly to such Hamiltonians. Here I will discuss a number of transformations, ranging from Jastrow factorisation in real systems, to Gutzwller factorisation in Hubbard models, which shows that such methods can lead to enormous gains in accuracy and efficiency. These methods allow explicit correlation to be introduced in strongly correlated electronic systems. Perspectives in future developments and applications will be provided.

Biography:

Ali Alavi is a director at the Max Planck Institute for Solid State Research as well as professor of theoretical chemistry at Cambridge. His primary current interest is to develop novel computational methodologies (mainly based on Monte Carlo ideas) for treating correlated electronic systems. Prior to this, he had worked on method development in density-functional theory, reaction mechanisms in surface and interface chemistry and electrochemistry. The difficulties density functional theory faces in these latter systems convinced him very early on (in the late 1990s) the need to move towards wavefunction based methods. He continues to believe this is very much way forward...

 

Mariana Rossi

Addressing the structure and dynamics of weakly-bonded interfaces

Abstract: Interfaces between different materials constitute the basis of many technological devices. Incorporating organic components within different architectures opens the path for creating more versatile interfaces with a wide range of properties at a reduced cost. However, the large conformational space that organic components can explore at finite temperatures and the inherent anharmonicity of their intra and intermolecular interactions brings further challenges to first-principles simulations. In this talk, I will discuss our recent efforts to address these challenges, based on developments within density functional theory an ab initio (path integral) molecular dynamics. I will present strategies for conformational space sampling of organic/inorganic interfaces, discuss the relationship between atomic and electronic structure, present techniques to include anharmonicity in vibrational fingerprints and machine learning tools to calculate these at reduced costs, and our recent methodological developments that allow the inclusion of quantum nuclear effects in high-dimensional systems (especially weakly bonded interfaces) using path integral molecular dynamics.

Biography

Mariana was born in Campinas, SP, Brazil, and studied Physics (bachelor and masters) in the University of São Paulo. During her junior project and master studies she started working with electronic structure theory and theories for charge transport under the supervision of Prof. Antônio José Roque da Silva and Prof. Adalberto Fazzio. She then moved to Berlin, Germany, to do her Ph.D. in the Fritz Haber Institute of the Max Planck Society, under the supervision of Prof. Volker Blum and Prof. Matthias Scheffler. In her Ph.D. she worked with structure determination of biomolecules from first-principles electronic structure methods. She focused on structure search and use of ab initio molecular dynamics to compute thermodynamical and vibrational properties of these systems. Her first post-doc was at the University of Oxford with Prof. David Manolopoulos, where she learned about path integral methods and approximate quantum dynamics, focusing her work on the inclusion of nuclear quantum effects in dynamical observables. She visited the group of Prof. Michele Ceriotti in École Polytechnique Fédèrale de Lausanne for one year, where she continued joining ab initio and path integral simulations for the calculations of thermodynamic properties of hydrogen-bonded systems. Since October 2016, she is back at the Fritz Haber Institute of the Max Planck Society in Berlin as an Otto-Hahn group leader. The group focuses on the study of H-bonded systems composed of biomolecules, organic molecules, and their interfaces with inorganic systems. Mariana was awarded fellowship for her master studies from the São Paulo Research Foundation (FAPESP), a Deutsche Forschungsgemeinschaft fellowship for post-doctoral studies, a Junior Research Fellowship at St. Edmund Hall in Oxford, and the Otto Hahn Award of the Max Planck Society.


Sotiris Xantheas

Advanced Computing, Mathematics and Data Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, MS K1-83, WA, 99352, USA
Department of Chemistry, University of Washington, Seattle, WA 98195, USA


The many-body expansion for aqueous systems revisited

We revisit the Many-Body Expansion (MBE) for water-water interactions by examining the effects of the basis set, including those resulting from the Basis Set Superposition Error (BSSE) correction, on the various terms for selected sizes of water clusters up to n = 21. The analysis is performed at the second order Møller-Plesset (MP2) perturbation theory with the family of augmented correlation consistent basis sets up to five zeta quality for the (H2O)n, n = 7, 10, 13, 16 and 21 clusters, for which we report either the complete MBE (n = 7, 10) or the one through the 6-body (n = 13) and the 5-body terms (n = 16, 21). Our results suggest that any sizeable contributions to the total cluster binding energy arising from the 5-body and larger terms are solely an artifact of the finite basis set. Indeed, all terms above the 4-body converge to practically zero at the Complete Basis Set (CBS) limit and this finding is accurately reproduced even with the smaller basis set of the series (aug-cc-pVDZ) once the BSSE correction is considered. The same level of theory (MP2/aug-cc-pVDZ, BSSE-corrected) also accurately reproduces the magnitude of the 3- and 4-body terms, for which we also find that the contributions of electron correlation are quite small. Our results unquestionably demonstrate that the MBE for water-water interactions vanishes monotonically with basis set size and can be safely truncated at the 4-body term once BSSE corrections are considered. We expect these findings to have important consequences in the pursuit of accurate many-body molecular dynamics simulations for aqueous systems.


*  This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacific Northwest National Laboratory (PNNL) is a multi-program national laboratory operated for DOE by Battelle.

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