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Computational Spectroscopy and Multireference Method Development

Computational Spectroscopy and Multireference Method Development

In our group, we focus on the development and application of methods for computational spectroscopy of molecules with a complicated electronic structure. Our primary interest is electronic spectroscopy of valence and core electrons of open-shell molecules. For such complicated systems, we use so-called multireference methods that solve the Schrödinger equation with multiple determinants on an equal footing. Typical applications of our approaches are UV/Vis spectra and X-ray absorption spectra of organic radicals and transition-metal complexes.

Benjamin Helmich-Paris

Dr. Benjamin Helmich-Paris

since 10/2020
Junior research group leader with at the Max-Planck-Institut für Kohlenforschung
01/2018
Researcher (Veni fellow (NWO)) at the Max-Planck-Institut für Kohlenforschung
11/2016
Researcher (Veni fellow (NWO)) at the Vrije Universiteit Amsterdam
05/2015
Post Doc with Lucas Visscher at the Vrije Universiteit Amsterdam (DFG Research fellow)
08/2014
Post Doc with Christof Hättig at the Ruhr-Universität Bochum
11/2010
Ph.D. student with Christof Hättig at the Ruhr University Bochum
07/2010
Diploma thesis with Joachim Sauer at the Humboldt-Universität zu Berlin
10/2008
Student assistant in the group of Joachim Sauer at the Humboldt-Universität zu Berlin
 

Research Topics:

Multi-Reference Linear Response Methods for Valence Electron Spectroscopy
Multi-Reference Linear Response Methods for Valence Electron Spectroscopy

Multi-Reference Linear Response Methods for Valence Electron Spectroscopy

Our current focal point is combining multireference (MR) methods with linear response (LR) theory. For the LR methods, excitation energies and intensities of the UV/Vis and electronic circular dichroism spectra are obtained from the solution of eigenvalue equations that contain the electronic Hessian. With the LR ansatz, a diversity of excitations can be described without augmenting the active space with spectroscopically relevant orbitals and electrons. In contrast to more established approaches based on state averaging (SA), also spectra of organic transition-metal complexes can be simulated that often include plenty of pi orbitals and electrons and can hardly by calculated with the SA approach. In the past, we have worked on an efficient CASSCF-LR implementation [18,19] that is available in the ORCA quantum chemistry package of Prof. Frank Neese since 2019. Ongoing and future research projects cover the inclusion of dynamic correlation to increase the accuracy. For this purpose, density functional theory and second-order perturbation theory will be used.

B. Helmich-Paris; CASSCF linear response calculations for large open-shell molecules. J. Chem. Phys. 150 (7), 174121 (2019). https://doi.org/10.1063/1.5092613

B. Helmich-Paris; Benchmarks for Electronically Excited States with CASSCF Methods. J. Chem. Theory Comput., 15, 4170-4179 (2019). https://doi.org/10.1021/acs.jctc.9b00325

Multi-Reference Linear Response Methods for Core Electron Spectroscopy
Multi-Reference Linear Response Methods for Core Electron Spectroscopy

Multi-Reference Linear Response Methods for Core Electron Spectroscopy

In one of our recent articles, we show how to adapt the CASSCF-LR method for simulating K-edges of X-ray absorption spectra.[22] The high-energy core-to-valence transitions can be reached either by a core-valence separation approximation or by a special algorithm for finding interior eigenvalues of the LR eigenvalue equations. First results for K-edges are encouraging. Of all available MR methods only the CASSCF-LR approach was able to describe the correct peak order for O-K-edges of MnO4- and ozone. As for the simulation of valence electronic spectra, including dynamic correlation will significantly increase the accuracy and will help us to gain new insights on the electronic structure of transition-metal containing molecules.

B. Helmich-Paris; Simulating X-ray absorption spectra with CASSCF linear response methods Int. J. Quantum Chem., in press(2020). https://arxiv.org/abs/2004.05845

Efficient and Accurate Approximations for Two-Electron Integrals
Efficient and Accurate Approximations for Two-Electron Integrals

Efficient and Accurate Approximations for Two-Electron Integrals

Our group also works on improving algorithms for computing two-electron integrals efficiently and accurately. Computing and processing two-electron integrals in Fock-matrix calculations is usually the most time-consuming step of Hartree-Fock (HF), DFT, and CASSCF calculations. The Coulomb (direct) and exchange part of the Fock matrix are well approximated with the resolution-of-the-identity approximation and semi-numerical chain-of-spheres (COSX) approach, respectively. In a recent project together with Prof. Frank Neese, Dr. Robert Izsak (Middlebury College), and Dr. Bernardo de Souza (FAccTs GmbH), we were able to make COSX calculations much faster by re-engineering the analytic electrostatic potential integrals. The errors of the numerical integration were also substantially reduced with newly developed grids where we have used machine learning. Our third generation COSX implementation allows us now to simulate UV/Vis spectra of large molecules using hybrid TDDFT and to compute binding energies of inter-molecular complexes using large basis sets of quadruple-zeta quality. Future work will be devoted to nuclear-displacement dependent properties like gradients and harmonic vibrational frequencies.

Robust Convergence of Self-Consistent-Field-Methods
Robust Convergence of Self-Consistent-Field-Methods

Robust Convergence of Self-Consistent-Field-Methods

One of our ongoing projects is the development of robust second-order methods for converging self-consistent field methods as HF, DFT, and CASSCF. In our recent work, we exploit the full electronic (augmented) Hessian (AH) in combination with trust-region (TR) methods to converge SCF energy calculations of molecules and clusters with a complicated electronic structure. For such systems, the standard direct inversion of the iterative subspace (DIIS) approach is problematic while our TRAH-SCF implementation in ORCA converges smoothly and reliably towards a local minimum. TRAH-SCF currently works for restricted and unrestricted HF and DFT and we plan to extend the approach to various MR methods.

 

 

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  • Dr. Benjamin Helmich-Paris

    Dr. Helmich-Paris, Benjamin

    +49 (0)208 306 - 2145

    helmichparis((atsign))kofo.mpg.de