arXiv subjects

Sarai Dery Folkestad

Publications and source records attributed to Sarai Dery Folkestad.

10 recordsLinked to original sources

arXiv2026

The RAS-CCSD model, with applications for open-shell molecules

The restricted active space coupled cluster singles and doubles (RAS-CCSD) model, and its pilot implementation is detailed. The approach offers a simple, pragmatic way to formulate a coupled cluster theory for open-shell and multireference systems. For ground state energies, errors (compared to FCI) are shown to be similar to those of unrestricted (spin-orbital) CCSD. For excited states, the errors of RAS-CCSD are generally larger, and adaptations to the methodology are needed to achieve an approach which is balanced for both ground and excited states. However, unlike unrestricted CCSD, the approach is spin-adapted and can be used to add dynamical correlation to a multideterminantal reference state.

physics.chem-ph↗

arXiv2026

Coupled cluster theory for multireference and open-shell molecules

We introduce a simple coupled cluster approach for multireference and open-shell systems. In the restricted active space coupled cluster framework, the cluster operator only includes excitations from doubly occupied to virtual orbitals, and this ensures the individual operators commute among each other. The ground and excited states are obtained by diagonalisation of the coupled cluster similarity transformed Hamiltonian in a restricted active space. We demonstrate the framework for the RAS-CCSD model, where single and double excitations are included in the cluster operator, and likewise for a complete active space. We consider systems with multireference character and open-shell systems in a spin-adapted manner. The approach captures significantly more dynamical correlation than without the coupled cluster similarity transformation.

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

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

Excitation Energies from the Entanglement Coupled Cluster Model for Doublets

We present excitation energies for molecular doublets from a spin-adapted formulation of coupled cluster singles and doubles theory. The entanglement coupled cluster approach represents an unconventional take on the notorious problem of spin adaptation for open-shell species. In this approach, the high-spin open-shell molecular system is coupled to non-interacting bath orbitals to form a total closed-shell system. In entanglement coupled cluster theory, many of the attractive features of the spin-adapted closed-shell coupled cluster is retained: an unambiguous definition of the cluster operator and a terminating Baker-Campbell-Hausdorff expansion. The result is a spin-adapted coupled cluster theory for open-shell species. The model produces excitation energies of a quality comparable to the closed-shell counterpart. Additionally, some ionized states that cannot be modeled accurately with the alternative equation-of-motion approach for ionized states, can be described with the entanglement coupled cluster singles and doubles model.

physics.chem-ph↗

arXiv2024

X-ray Absorption Spectra for Aqueous Ammonia and Ammonium: Quantum Mechanical versus Molecular Mechanical Embedding Schemes

The X-ray absorption (XA) spectra of aqueous ammonia and ammonium are computed using a combination of coupled cluster singles and doubles (CCSD) with different quantum mechanical and molecular mechanical embedding schemes. Specifically, we compare frozen Hartree--Fock (HF) density embedding, polarizable embedding (PE), and polarizable density embedding (PDE). Integrating CCSD with frozen HF density embedding is possible within the CC-in-HF framework, which circumvents the conventional system-size limitations of standard coupled cluster methods. We reveal similarities between PDE and frozen HF density descriptions, while PE spectra differ significantly. By including approximate triple excitations, we also investigate the effect of improving the electronic structure theory. The spectra computed using this approach show an improved intensity ratio compared to CCSD-in-HF. Charge transfer analysis of the excitations shows the local character of the pre-edge and main-edge, while the post-edge is formed by excitations delocalized over the first solvation shell and beyond.

physics.chem-ph↗

arXiv2023

Understanding X-ray absorption in liquid water: triple excitations in multilevel coupled cluster theory

We present the first successful application of the coupled cluster approach to simulate the X-ray absorption (XA) spectrum of liquid water. The system size limitations of standard coupled cluster theory are overcome by employing a newly developed coupled cluster method for large molecular systems. This method combines coupled cluster singles, doubles, and perturbative triples in a multilevel framework (MLCC3-in-HF) and is able to describe the delicate nature of intermolecular interactions in liquid water. Using molecular geometries from state-of-the-art path-integral molecular dynamics, we obtain excellent agreement with experimental spectra. Additionally, we show that an accurate description of the electronic structure within the first solvation shell is sufficient to model the XA spectrum of liquid water. Furthermore, we present a rigorous charge transfer analysis with unprecedented reliability, achieved through MLCC3-in-HF. This analysis aligns with previous studies regarding the character of the prominent features of the spectrum.

physics.chem-ph↗

arXiv2023

Entanglement Coupled Cluster Theory: Exact Spin-Adaptation

We present a novel approach to spin-adapted coupled cluster theory. This approach is based on the entanglement of an open-shell molecule with electrons in a non-interacting bath; together they form a closed-shell state. For the total system, electron correlation can be added with the standard spin-adapted closed-shell coupled cluster formalism. A projection operator, which enforces conditions on the electrons in the bath, is used to obtain the desired state of the molecular system. This entanglement coupled cluster theory is detailed and proof-of-concept calculations for doublet states are reported. The approach is extendable to open-shell systems with other values of the total spin.

physics.chem-ph↗

arXiv2023

New Implementation of an Equation-of-Motion Coupled-Cluster Damped-Response Framework with Illustrative Applications to Resonant Inelastic X-ray Scattering

We present an implementation of a damped response framework for calculating resonant inelastic X-ray scattering (RIXS) at the equation-of-motion coupled cluster singles and doubles (CCSD) and second-order approximate coupled cluster singles and doubles (CC2) levels of theory in the open-source program $e^T$. This framework lays the foundation for future extension to higher excitation methods (notably, the coupled cluster singles and doubles with perturbative triples, CC3) and to multilevel approaches. Our implementation adopts a fully relaxed ground state, and different variants of the core-valence separation projection technique to address convergence issues. Illustrative results are compared with those obtained within the frozen-core core-valence separated approach, available in Q-Chem, as well as with experiment. The performance of the CC2 method is evaluated in comparison with that of CCSD. It is found that, while the CC2 method is noticeably inferior to CCSD for X-ray absorption spectra, the quality of the CC2 RIXS spectra is often comparable to that of the CCSD level of theory, when the same valence excited states are probed. Finally, we present preliminary RIXS results for a solvated molecule in aqueous solution.

physics.chem-ph↗