Stefan Knecht

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Knecht

First Name

Stefan

ORCID

0000-0001-9818-2372

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Publications 1 - 10 of 20

The Dalton quantum chemistry program system
Aidas, Kestutis; Angeli, Celestino; Bak, Keld L.; et al. (2014)
Wiley Interdisciplinary Reviews. Computational Molecular Science
Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree–Fock, Kohn–Sham, multiconfigurational self-consistent-field, Møller–Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge-origin-invariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.
Correlated Dirac-Coulomb-Breit multiconfigurational self-consistent-field methods
Hoyer, Chad E.; Lu, Lixin; Hu, Hang; et al. (2023)
The Journal of Chemical Physics
The fully correlated frequency-independent Dirac-Coulomb-Breit Hamiltonian provides the most accurate description of electron-electron interaction before going to a genuine relativistic quantum electrodynamics theory of many-electron systems. In this work, we introduce a correlated Dirac-Coulomb-Breit multiconfigurational self-consistent-field method within the frameworks of complete active space and density matrix renormalization group. In this approach, the Dirac-Coulomb-Breit Hamiltonian is included variationally in both the mean-field and correlated electron treatment. We also analyze the importance of the Breit operator in electron correlation and the rotation between the positive-and negative-orbital space in the no-virtual-pair approximation. Atomic fine-structure splittings and lanthanide contraction in diatomic fluorides are used as benchmark studies to understand the contribution from the Breit correlation.
Charge-Transfer-Induced Predissociation in Rydberg States of Molecular Cations: MgAr+
Wehrli, Dominik; Génévriez, Matthieu; Knecht, Stefan; et al. (2021)
The Journal of Physical Chemistry A
Very little is known about the Rydberg states of molecular cations, i.e., Rydberg states having a doubly charged ion core. With the example of MgAr+, we present general features of the structure and dynamics of the Rydberg states of molecular cations, which we find are subject to the process of charge-transfer-induced predissociation. Our study focuses on the spectrum of low-n Rydberg states with potential-energy functions associated with the Mg+(3d and 4s) + Ar(S-1(0)) dissociation asymptotes. In particular, we have recorded spectra of the 3d pi(Omega') (Omega' = 1/2, 3/2) Rydberg states, extending from the lowest (v' = 0) vibrational levels to their dissociation limits. This spectral range encompasses the region where the onset of predissociation by interaction with the mostly repulsive (2)Sigma and (2)Pi charge-transfer states associated with the Mg(3s(2)) + Ar+(P-2(1/2,3/2)) dissociation asymptotes is observed. This interaction leads to very strong perturbations of the 3d pi Rydberg states of MgAr+, revealed by vibrational progressions exhibiting large and rapid variations of the vibrational intervals, line widths, and spin-orbit splittings. We attribute the anomalous sign and magnitude of the spin-orbit coupling constant of the 3dp state to the interaction with a (2)Pi Rydberg state correlating to the Mg+(4p) + Ar(S-1(0)) dissociation limit. To analyze our spectra and elucidate the underlying process of charge-transfer-induced predissociation, we implemented a model that allowed us to derive the potential-energy functions of the charge-transfer states and to quantitatively reproduce the experimental results. This analysis characterizes the main features of the dynamics of the Rydberg series converging to the ground state of MgAr2+. We expect that the results and analysis reported here are qualitatively valid for a broader range of singly charged molecular cations, which are inherently prone to charge-transfer interactions.
OpenMolcas: From Source Code to Insight
Fdez. Galván, Ignacio; Vacher, Morgane; Alavi, Ali; et al. (2019)
Journal of Chemical Theory and Computation
Quantum Information-Assisted Complete Active Space Optimization (QICAS)
Ding, Lexin; Knecht, Stefan; Schilling, Christian (2023)
The Journal of Physical Chemistry Letters
We propose an effective quantum information-assisted complete active space optimization scheme (QICAS). What sets QICAS apart from other correlation-based selection schemes is (i) the use of unique measures from quantum information that assess the correlation in electronic structures in an unambiguous and predictive manner and (ii) an orbital optimization step that minimizes the correlation discarded by the active space approximation. Equipped with these features, QICAS yields, for smaller correlated molecule, sets of optimized orbitals with respect to which the complete active space configuration interaction energy reaches the corresponding complete active space self-consistent field (CASSCF) energy within chemical accuracy. For more challenging systems such as the chromium dimer, QICAS offers an excellent starting point for CASSCF by greatly reducing the number of iterations required for numerical convergence. Accordingly, our study validates a profound empirical conjecture: the energetically optimal nonactive spaces are predominantly those that contain the least entanglement.
Density matrix renormalization group with efficient dynamical electron correlation through range separation
Hedegård, Erik D.; Knecht, Stefan; Kielberg, Jesper S.; et al. (2015)
The Journal of Chemical Physics
The variational quantum eigensolver self-consistent field method within a polarizable embedded framework
Kjellgren, Erik Rosendahl; Reinholdt, Peter; Fitzpatrick, Aaron; et al. (2024)
The Journal of Chemical Physics
We formulate and implement the Variational Quantum Eigensolver Self Consistent Field (VQE-SCF) algorithm in combination with polarizable embedding (PE), thereby extending PE to the regime of quantum computing. We test the resulting algorithm, PE-VQE-SCF, on quantum simulators and demonstrate that the computational stress on the quantum device is only slightly increased in terms of gate counts compared to regular VQE-SCF. On the other hand, no increase in shot noise was observed. We illustrate how PE-VQE-SCF may lead to the modeling of real chemical systems using a simulation of the reaction barrier of the Diels-Alder reaction between furan and ethene as an example.
Modern quantum chemistry with [Open]Molcas
Aquilante, Francesco; Baiardi, Alberto; Freitag, Leon; et al. (2020)
The Journal of Chemical Physics
MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree–Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.
Quantum correlations in molecules: from quantum resourcing to chemical bonding
Ding, Lexin; Knecht, Stefan; Zimborás, Zoltán; et al. (2023)
Quantum Science and Technology
The second quantum revolution is all about exploiting the quantum nature of atoms and molecules to execute quantum information processing tasks. To boost this growing endeavor and by anticipating the key role of quantum chemistry therein, our work establishes a framework for systematically exploring, quantifying and dissecting correlation effects in molecules. By utilizing the geometric picture of quantum states we compare-on a unified basis and in an operationally meaningful way-total, quantum and classical correlation and entanglement in molecular ground states. To unlock and maximize the quantum informational resourcefulness of molecules an orbital optimization scheme is developed, leading to a paradigm-shifting insight: a single covalent bond equates to the entanglement 2ln(2)
Complete characterization of the 3p Rydberg complex of a molecular ion: MgAr+
Wehrli, Dominik; Génévriez, Matthieu; Knecht, Stefan; et al. (2020)
The Journal of Chemical Physics

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Stefan Knecht

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