Tilmann Bodenstein

• Cezar, Henrique Musseli; Bodenstein, Tilmann; Sveinsson, Henrik Andersen; Ledum, Morten; Reine, Simen & Bore, Sigbjørn Løland (2025). Learning atomic forces from uncertainty-calibrated adversarial attacks. npj Computational Materials. 11. doi: 10.1038/s41524-025-01703-5. Fulltekst i vitenarkiv Vis sammendrag

  Adversarial approaches, which intentionally challenge machine learning models by generating difficult examples, are increasingly being adopted to improve machine learning interatomic potentials (MLIPs). While already providing great practical value, little is known about the actual prediction errors of MLIPs on adversarial structures and whether these errors can be controlled. We propose the Calibrated Adversarial Geometry Optimization (CAGO) algorithm to discover adversarial structures with user-assigned errors. Through uncertainty calibration, the estimated uncertainty of MLIPs is unified with real errors. By performing geometry optimization for calibrated uncertainty, we reach adversarial structures with the user-assigned target MLIP prediction error. Integrating with active learning pipelines, we benchmark CAGO, demonstrating stable MLIPs that systematically converge structural, dynamical, and thermodynamical properties for liquid water and water adsorption in a metal-organic framework within only hundreds of training structures, where previously many thousands were typically required.

• Bodenstein, Tilmann; Heimermann, Andreas; Fink, Karin & Wüllen, Christoph van (2021). Development and Application of a Complete Active Space Spin-Orbit Configuration Interaction Program Designed for Single Molecule Magnets. ChemPhysChem. ISSN 1439-4235. doi: 10.1002/cphc.202100648. Fulltekst i vitenarkiv Vis sammendrag

  We present a spin-orbit configuration interaction program which has been tailored for the description of the magnetic properties of polynuclear metal complexes with partially filled d- and f-shells. The spin-orbit operators are directly included in the configuration interaction program based on Slater-determinants. The lowest states are obtained by a Block-Davidson-type diagonalisation. The usage of localised active orbitals enables the construction of start vectors from tensor products of single-center wave functions that already include spin-orbit interaction. This allows for an analysis of the role and the interplay of the different metal centres. Furthermore, in case of weak coupling of the metal centres these tensor products are already close to the final wave functions ensuring fast convergence. In combination with a two-layer hybrid parallelisation, this makes the program highly efficient. Based on the spin-orbit coupled wave functions, magnetic D-tensors, g-tensors and temperature-dependent susceptibilities can be calculated. The applicability and performance of the program is shown exemplarily on a trinuclear transition metal (CoIIVIICoII) complex.

• Pedersen, Thomas Bondo; Kristiansen, Håkon Emil; Bodenstein, Tilmann; Kvaal, Simen & Schøyen, Øyvind Sigmundson (2021). Interpretation of Coupled-Cluster Many-Electron Dynamics in Terms of Stationary States. Journal of Chemical Theory and Computation. ISSN 1549-9618. 17(1), s. 388–404. doi: 10.1021/acs.jctc.0c00977. Fulltekst i vitenarkiv Vis sammendrag

  We demonstrate theoretically and numerically that laser-driven many-electron dynamics, as described by bivariational time-dependent coupled-cluster theory, may be analyzed in terms of stationary-state populations. Projectors heuristically defined from linear response theory and equation-of-motion coupled cluster theory are proposed for the calculation of stationary-state populations during interaction with laser pulses or other external forces, and conservation laws of the populations are discussed. Numerical tests of the proposed projectors, involving both linear and nonlinear optical processes for the He and Be atoms, and for the LiH, CH+, and LiF molecules, show that the laser-driven evolution of the stationary-state populations at the coupled-cluster singles-and-doubles (CCSD) level is very close to that obtained by ful configuration-interaction theory provided all stationary states actively participating in the dynamics are sufficiently well approximated. When double-excited states are important for the dynamics, the quality of the CCSD results deteriorate. Observing that populations computed from the linear-response projector may show spurious small-amplitude, high-frequency oscillations, the equation-of-motion projector emerges as the most promising approach to stationary-state populations.

• Bodenstein, Tilmann & Kvaal, Simen (2020). A state-specific multireference coupled-cluster method based on the bivariational principle. Journal of Chemical Physics. ISSN 0021-9606. 153(2). doi: 10.1063/5.0009429. Fulltekst i vitenarkiv Vis sammendrag

  A state-specific multireference coupled-cluster (MRCC) method based on Arponen’s bivariational principle is presented, the bivar-MRCC method. The method is based on single-reference theory and therefore has a relatively straightforward formulation and modest computational complexity. The main difference from established methods is the bivariational formulation, in which independent parameterizations of the wave function (ket) and its complex conjugate (bra) are made. Importantly, this allows manifest multiplicative separability of the state (exact in the extended bivar-MRECC version of the method and approximate otherwise), and additive separability of the energy, while preserving polynomial scaling of the working equations. A feature of the bivariational principle is that the formal bra and ket references can be included as bivariational parameters, which eliminates much of the bias toward the formal reference. A pilot implementation is described, and extensive benchmark calculations on several standard problems are performed. The results from the bivar-MRCC method are comparable to established state-specific multireference methods. Considering the relative affordability of the bivar-MRCC method, it may become a practical tool for non-experts.

• Kvaal, Simen; Laestadius, Andre & Bodenstein, Tilmann (2020). Guaranteed convergence for a class of coupled-cluster methods based on Arponen's extended theory. Molecular Physics. ISSN 0026-8976. 118(19-20). doi: 10.1080/00268976.2020.1810349. Fulltekst i vitenarkiv Vis sammendrag

  A wide class of coupled-cluster methods is introduced, based on Arponen's extended coupled-cluster theory. This class of methods is formulated in terms of a coordinate transformation of the cluster operators. The mathematical framework for the error analysis of coupled-cluster methods based on Arponen's bivariational principle is presented, in which the concept of local strong monotonicity of the flipped gradient of the energy is central. A general mathematical result is presented, describing sufficient conditions for coordinate transformations to preserve the local strong monotonicity. The result is applied to the presented class of methods, which include the standard and quadratic coupled-cluster methods, and also Arponen's canonical version of extended coupled-cluster theory. Some numerical experiments are presented, and the use of canonical coordinates for diagnostics is discussed.

• Bodenstein, Tilmann & Eichhofer, Andreas (2019). Magnetic anisotropy in trigonal planar Fe(ii) bis(trimethylsilyl)amido complexes of the type [Fe{N(SiMe3)2}2L]—experiment and theory†. Dalton Transactions. ISSN 1477-9226. 48(41), s. 15699–15712. doi: 10.1039/c9dt01702j. Fulltekst i vitenarkiv
• Schieschke, Nils; Bodenstein, Tilmann & Höfener, Sebastian (2019). Frozen-density embedding employing configuration interaction as a subsystem method. Molecular Physics. ISSN 0026-8976. 118(12). doi: 10.1080/00268976.2019.1665726. Fulltekst i vitenarkiv

Se alle arbeider i NVA

• Kvaal, Simen; Bodenstein, Tilmann; Laestadius, Andre & Reinhold, Schneider (2023). Development of multireference coupled-cluster methods from single-state and multi-state bivariational principles. Fulltekst i vitenarkiv
• Bodenstein, Tilmann & Kvaal, Simen (2020). A multireference coupled-cluster method based on the bivariational principle. Fulltekst i vitenarkiv

Se alle arbeider i NVA