arXiv subjects

Ida-Marie Høyvik

Publications and source records attributed to Ida-Marie Høyvik.

7 recordsLinked to original sources

arXiv2026

Different Environments in Quantum Statistical Mechanics: To $β$ or not to $(k_{\mathrm{B}} T)^{-1}$

We piece together textbook material to re-derive the parameter $β$ entering reduced density operators from quantum statistical mechanics. By re-deriving $β$, we show that the content of $β$ depends on the particular application of statistical mechanics. We focus on two different applications, namely, electronic-structure theory (new) and thermodynamics (standard). Specifically, we show that $β$ becomes proportional to the inverse Fermi energy, when the electronic states of a system interact with the infinitely many valence electrons in a metal. On the other hand, when the environment is a heat bath, $β$ takes the well-known form of inverse temperature. To highlight the importance of using the correct form of $β$, we explore statistical descriptions of a potassium atom adsorbed on a gold surface, where the valence electrons of gold represent the environment of potassium. By treating $β$ as the inverse Fermi energy, we are able to qualitatively reproduce the fractional charging of potassium. In contrast, if we interpret $β$ as the inverse temperature, extremely high and unphysical temperatures are required to reproduce the same qualitative picture. Thereby, we illustrate the fallacy of not separating the mathematical framework of quantum statistical mechanics from its pervasive thermodynamic application.

physics.chem-ph↗

arXiv2025

The role of charge resonances in the benzene dimer

Modern electronic-structure theory defines dispersion interactions as connected intramonomer excitations. Using this definition, dispersion contributions have been shown in literature to be large relative to other contributions at van der Waals distances for the ground state benzene dimer. However, are the dispersion contributions sufficient to describe its potential energy surface? In this paper, we show the importance of charge resonances for the shape of the potential energy surface of the stacked benzene dimer. Charge resonances is a colloquial term for the presence of ion-pair configurations in the electronic wave function, and they represent a charge delocalization between the benzene molecules. We show that the ion-pair configurations, generated from connected intra- and intermonomer excitations, have a significant impact on the potential energy curves as functions of parallel displacement, as well as intramonomer separation. For parallel displacement, the energy minimum shifts approximately 2 Å toward greater displacement if ion-pair configurations are not included. Hence, to understand the non-covalent bonding in the benzene dimer two mechanisms must be taken into account: dispersion interaction and charge resonances.

physics.chem-ph↗

arXiv2025

A Reusable Library for Second-Order Orbital Optimization Using the Trust Region Method

We present a reusable, open-source software implementation of the second-order trust region algorithm in the new OpenTrustRegion library. We apply the implementation to the general-purpose optimization of molecular orbitals in various contexts within electronic-structure theory. Our permissibly licensed implementation can be included in any software package, be it free and open-source, academically licensed closed-source, or commercial. Detailing the implementation in OpenTrustRegion, we present a review of the theory behind trust region-based methods alongside various extensions. We demonstrate the robustness and efficiency of our optimization library with extensive benchmarks for self-consistent field calculations, orbital localization, as well as orbital symmetrization tasks, featuring challenging and pathological systems.

physics.chem-ph↗

arXiv2025

eT 2.0: An efficient open-source molecular electronic structure program

The eT program is an open-source electronic structure program with emphasis on performance and modularity. As its name suggests, the program features extensive coupled cluster capabilities, performing well compared to other electronic structure programs, and, in some cases, outperforming commercial alternatives. However, eT is more than a coupled cluster program; other models based on wave function theory (such as full and reduced space configuration interaction and a variety of self-consistent field models) and density functional theory are supported. The second major release of the program, eT 2.0, has specialized functionality for strong light-matter coupling conditions. In addition, it includes a wide range of optimizations and algorithmic improvements, as well as new capabilities for exploring potential energy surfaces and for modeling experiments in the UV and X-ray regimes. Molecular gradients are now available at the coupled cluster level, and high-accuracy spectroscopic simulations are available at reduced computational cost within the multilevel coupled cluster and multiscale frameworks. We present the modifications to the program since its first major release, eT 1.0, highlighting some notable new features and demonstrating the performance of the new version relative to the first release and to other established electronic structure programs.

physics.chem-ph↗

arXiv2025

An orthogonal electronic state view on charge delocalization and transfer

We present a configuration interaction (CI) framework which serves quantitative and conceptual purposes for charge delocalization and electron transfer processes in molecular systems. The electronic Hamiltonian is expressed in a basis of charge-localized determinants and used to independently generate adiabatic CI states and charge-localized CI states, both of which are unambiguously defined through a diagonalization procedure. The CI framework offers a simple interpretation of adiabatic states as resonance hybrids of different electron distributions, providing a simple picture for discussing charge-delocalization in chemical bonding. The charge-localized states serve as a convenient orthogonal representation of initial and final states in electron transfer processes, and provides an unambiguous definition of their electronic coupling. These two models enable an analysis of the water dimer hydrogen bond. We demonstrate that although the overall charge delocalization is small, the occurrence of particular ionic contributions are crucial to get the correct electronic description.

physics.chem-ph↗

arXiv2025

Quantum Statistical Mechanics of Electronically Open Molecules: Reduced Density Operators

We present a reduced density operator for electronically open molecules by explicitly averaging over the environmental degrees of freedom of the composite Hamiltonian. Specifically, we include the particle-number non-conserving (particle-breaking) interactions responsible for the sharing of electrons between the molecule and the environment, which are neglected in standard formulations of quantum statistical mechanics. We propose an unambiguous definition of the partial trace operation in the composite fermionic Fock space based on composite states in a second quantization framework built from a common orthonormal set of orbitals. Thereby, we resolve the fermionic partial trace ambiguity. The common orbital basis is constructed by spatial localization of the full orbital space, in which the full composite Hamiltonian naturally partitions into a molecule Hamiltonian, an environment Hamiltonian, and an interaction Hamiltonian. The new reduced density operator is based on the approximation of commutativity between the subsystem Hamiltonians (i.e., molecule and environment Hamiltonians) and the interaction Hamiltonian, which we show corresponds to excluding certain electron transfer channels and neglecting electron transfer relaxation effects. The reduced density operator can be viewed as a generalization of the grand canonical density operator. We are prompted to define the generalized chemical potential, which aligns with the standard interpretation of the chemical potential, apart from the possibility of fractional rather than strictly integer electron transfer in our framework. In contrast to standard approaches, our framework enables an explicit consideration of the electron occupancy in the environment at any level of theory, irrespective of the model used to describe the molecule.

physics.chem-ph↗

arXiv2020

eT 1.0: an open source electronic structure program with emphasis on coupled cluster and multilevel methods

The eT program is an open source electronic structure package with emphasis on coupled cluster and multilevel methods. It includes efficient spin adapted implementations of ground and excited singlet states, as well as equation of motion oscillator strengths, for CCS, CC2, CCSD, and CC3. Furthermore, eT provides unique capabilities such as multilevel Hartree-Fock and multilevel CC2, real-time propagation for CCS and CCSD, and efficient CC3 oscillator strengths. With a coupled cluster code based on an efficient Cholesky decomposition algorithm for the electronic repulsion integrals, eT has similar advantages as codes using density fitting, but with strict error control. Here we present the main features of the program and demonstrate its performance through example calculations. Because of its availability, performance, and unique capabilities, we expect eT to become a valuable resource to the electronic structure community.

physics.chem-ph↗