audunsh

• Odden, Tor Ole Bigton & Hansen, Audun Skau (2025). Physics Intuition About Vectors Can Help Us Understand Generative Artificial Intelligence. The Physics Teacher. ISSN 0031-921X. 63(3), s. 173–176. doi: 10.1119/5.0149555. Fulltekst i vitenarkiv
• Olsen, Christine; Wang, Chencheng; Abadpour, Shadab; Lundanes, Elsa; Hansen, Audun Skau & Skottvoll, Frøydis Sved [Vis alle 8 forfattere av denne artikkelen] (2022). Determination of insulin secretion from stem cell-derived islet organoids with liquid chromatography-tandem mass spectrometry. Journal of chromatography. B. ISSN 1570-0232. 1215. doi: 10.1016/j.jchromb.2022.123577. Fulltekst i vitenarkiv Vis sammendrag

  Organoids are laboratory-grown 3D organ models, mimicking human organs for e.g. drug development and personalized therapy. Islet organoids (typically 100–200 µm), which can be grown from the patient́s own cells, are emerging as prototypes for transplantation-based therapy of diabetes. Selective methods for quantifying insulin production from islet organoids are needed, but sensitivity and carry-over have been major bottlenecks in previous efforts. We have developed a reverse phase liquid chromatography-tandem mass spectrometry (RPLC-MS/MS) method for studying the insulin secretion of islet organoids. In contrast to our previous attempts using nano-scale LC columns, conventional 2.1 mm inner diameter LC column (combined with triple quadrupole mass spectrometry) was well suited for sensitive and selective measurements of insulin secreted from islet organoids with low microliter-scale samples. Insulin is highly prone to carry-over, so standard tubings and injector parts were replaced with shielded fused silica connectors. As samples were expected to be very limited, an extended Box-Behnken experimental design for the MS settings was conducted to maximize performance. The finale method has excellent sensitivity, accuracy and precision (limit of detection: ≤0.2 pg/µL, relative error: ≤±10%, relative standard deviation: <10%), and was well suited for measuring 20 µL amounts of Krebs buffer containing insulin secreted from islet organoids.

• Hansen, Audun Skau; Aurbakken, Einar & Pedersen, Thomas Bondo (2021). Smooth potential-energy surfaces in fragmentation-based local correlation methods for periodic systems. Molecular Physics. ISSN 0026-8976. 119(9). doi: 10.1080/00268976.2021.1896046. Fulltekst i vitenarkiv Vis sammendrag

  Local approximations facilitate the application of post-Hartree–Fock methods in the condensed phase, but simultaneously introduce errors leading to discontinuous potential-energy surfaces. In this work, we explore how these discontinuities arise in periodic systems, their implications, and possible ways of controlling them. In addition, we present a fully periodic Divide-Expand-Consolidate second-order Møller–Plesset approach using an attenuated resolution-of-the-identity approximation for the electron repulsion integrals and a convenient class to handle translation-symmetric tensors in block-Toeplitz format.

• Hansen, Audun Skau; Baardsen, Gustav; Rebolini, Elisa; Maschio, Lorenzo & Pedersen, Thomas Bondo (2020). Representation of the virtual space in extended systems – a correlation energy convergence study. Molecular Physics. ISSN 0026-8976. doi: 10.1080/00268976.2020.1733118. Fulltekst i vitenarkiv Vis sammendrag

  We present an investigation of the convergence behaviour of the local second-order Møller-Plesset perturbation theory (MP2) correlation energy toward the canonical result for three insulating crystals with either projected atomic orbitals (PAOs) or various orthonormal representations of the virtual orbital space. Echoing recent results for finite molecular systems, we find that significantly fewer PAOs than localised orthonormal virtual orbitals are required to reproduce the canonical correlation energy. We find no clear-cut correlation between conventional measures of orbital locality and the ability of the representation to span the excitation space of local domains. We show that the PAOs of the reference unit cell span parts of the excitation space that can only be reached with distant local orthonormal virtual orbitals.

• Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2018). Divide-Expand-Consolidate Second-Order Møller-Plesset Theory with Periodic Boundary Conditions. Journal of Chemical Theory and Computation. ISSN 1549-9618. 14(5), s. 2427–2438. doi: 10.1021/acs.jctc.8b00021. Fulltekst i vitenarkiv Vis sammendrag

  We present a generalization of the divide–expand–consolidate (DEC) framework for local coupled-cluster calculations to periodic systems and test it at the second-order Møller–Plesset (MP2) level of theory. For simple model systems with periodicity in one, two, and three dimensions, comparisons with extrapolated molecular calculations and the local MP2 implementation in the Cryscor program show that the correlation energy errors of the extended DEC (X-DEC) algorithm can be controlled through a single parameter, the fragment optimization threshold. Two computational bottlenecks are identified: the size of the virtual orbital spaces and the number of pair fragments required to achieve a given accuracy of the correlation energy. For the latter, we propose an affordable algorithm based on cubic splines interpolation of a limited number of pair-fragment interaction energies to determine a pair cutoff distance in accordance with the specified fragment optimization threshold.

Se alle arbeider i NVA

• Hansen, Audun Skau (2022). Shuffleboard og kjemi. Kjemi. ISSN 0023-1983. Fulltekst i vitenarkiv
• Hansen, Audun Skau; Helgaker, Trygve & Laestadius, Andre (2022). En introduksjon til kvantekjemi / Samarbeid med Talentsenter i realfag. Fulltekst i vitenarkiv
• Hansen, Audun Skau (2022). Virtuell kjemilab / Ungforsk. Fulltekst i vitenarkiv
• Haraldsrud, Andreas & Hansen, Audun Skau (2021). Exploring chemistry with programming and numerical experiments. Fulltekst i vitenarkiv
• Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2018). Error-controlled MP2 for periodic molecular systems. Fulltekst i vitenarkiv
• Hansen, Audun Skau; Baardsen, Gustav; Maschio, Lorenzo & Pedersen, Thomas Bondo (2018). Locality and sparsity in local correlation calculations. Fulltekst i vitenarkiv
• Baardsen, Gustav; Rebolini, Elisa; Hansen, Audun Skau; Maschio, Lorenzo; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2018). The divide-expand-consolidate method for extended systems. Fulltekst i vitenarkiv
• Hansen, Audun Skau (2017). Stochastic optimization of block Toeplitz matrices. Fulltekst i vitenarkiv
• Hansen, Audun Skau (2017). Stochastic optimization of block-Toeplitz matrices. Fulltekst i vitenarkiv
• Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2017). Local coupled cluster methods for periodic systems. Fulltekst i vitenarkiv
• Leikanger, Karl R.; Rebolini, Elisa; Hansen, Audun Skau; Baardsen, Gustav & Pedersen, Thomas Bondo (2016). Hartree-Fock calculations using a priori Wannier orbitals for solids. Fulltekst i vitenarkiv
• Hansen, Audun Skau; Pedersen, Thomas Bondo; Helgaker, Trygve & Kvaal, Simen (2021). Local correlation methods for infinite systems. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Quantum Chemistry constitutes an extensive framework for accurately simulating the electronic wavefunction of atoms and molecules. The same framework may in principle be applied to the domain of periodic structures such as crystals, but is in practice severely limited by the infinite nature of these structures in conjunction with the computational complexity of quantum chemical methods. In his thesis, the candidate utilizes a mathematical structure known as bi-infinite block-Toeplitz matrices in order to smoothly transition between the molecular and periodic realm. Furthermore, he extends the divide-expand-consolidate methods originally devised for molecules to the periodic case, and demonstrates that this procedure can reduce the computational scaling of the simulation while retaining systematic control over the error.

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