arXiv subjects

Elvar Örn Jónsson

Publications and source records attributed to Elvar Örn Jónsson.

11 recordsLinked to original sources

arXiv2026

DFT GGA based datasets for H$_2$O potential energy surfaces, permanent moment and polarizability tensors

We present in this article data on the computed potential energy surface, permanent moment surface (from dipole to hexadecapole) and polarizability surface (from dipole-dipole to quadrupole-quadrupole) for the H$_2$O molecule, calculated at the level of the generalized gradient approximation (GGA) in Kohn-Sham density functional theory (KS-DFT) in the grid-based projector augmented wave code GPAW, using the charge perturbation code C-pol. The GGA-based functionals are PBE, RPBE and BLYP. The monomer surfaces are composed of systematic, symmetry-reduced deformations of the H$_2$O monomer and comprise 10,000 configurations per functional, with traceless Cartesian tensor moment values reported as data strings in the xyz file format. In addition, we provide cluster datasets for n-H$_2$O aggregates (n = 2-6), sampled from liquid-like configurations and computed with the same KS-DFT settings, where each extended xyz file encodes the total interaction energy of the cluster in the comment line. Finally, ab initio molecular dynamics (AIMD) trajectories of periodic liquid water boxes, containing between 25 to 150 H$_2$O molecules at ambient conditions, are included, furnishing time series of atomic configurations and energies that are consistent with the monomer and cluster data and suitable for validating classical and machine-learned water models.

physics.chem-ph↗

arXiv2026

Excited-state Properties Beyond the Excitation Energy from Orbital-Optimized Density Functional Calculations I: Dipole Moments of Rydberg States

Rydberg excited states are challenging to describe due to their highly diffuse character. Orbital-optimized density functional calculations typically provide more accurate values of the excitation energy of Rydberg states than time-dependent density functional theory approaches. However, the reliability of orbital-optimized methods for properties of Rydberg excited states such as the dipole moment remains much less explored, with existing benchmarks largely limited to the lowest excited states. Here, orbital-optimized density functional calculations with a plane-wave basis set are used to compute the dipole moment of several Rydberg states of a set of small molecules. Plane waves provide a flexible representation of diffuse Rydberg orbitals, overcoming limitations of commonly used atomic orbitals basis sets. Due to overconfinement of the Rydberg orbitals, a single-augmented atomic basis set yields a magnitude of the dipole moment that disagrees with the plane-wave calculations, even when the corresponding excitation energy is in good agreement. For the most diffuse states, the orientation of the dipole moment predicted by the atomic orbitals basis set can also be incorrect, and discrepancies with plane waves calculations persist even when extra augmented diffuse functions are added. The generalized gradient approximation functional PBE used in combination with the plane-wave representation of the orbitals gives good agreement with higher-level coupled-cluster calculations performed with sufficiently diffuse basis sets, when the latter are available. The hybrid functional PBE0 further improves the results, while PBE with globally scaled explicit Perdew-Zunger self-interaction correction generally leads to larger errors and an overestimation of the dipole moment, despite restoring the correct asymptotic $-1/r$ dependence of the effective Kohn--Sham potential.

physics.chem-ph↗

arXiv2026

Excited-state Properties Beyond the Excitation Energy from Orbital-Optimized Density Functional Calculations II: Absorption Spectra

Absorption spectra up to 10 eV are calculated for a set of small molecules using a variational density functional approach in which the orbitals are optimized for each excited state. A plane-wave basis set is employed to accurately describe diffuse Rydberg excitations, while the transition dipole moment is evaluated from nonorthogonal Kohn--Sham determinants within the projector augmented wave formalism. Comparison with higher-level coupled-cluster results shows that orbital-optimized calculations provide a good description of oscillator strengths for states with predominantly single-configurational character, even with the generalized gradient approximation functional PBE. The inclusion of exact exchange and self-interaction correction further improves the results, with the latter yielding the smallest errors (mean absolute relative error of $\sim$25%). In contrast, large errors for all functionals are found for multi-configurational states, due to an inability of the approach to describe the multi-configurational character. Instead, the nonorthogonality between the ground and excited states is not found to be a significant source of error. These results establish the performance of orbital-optimized density functional calculations for absorption spectra of small molecules and highlight the need for extensions that combine state-specific orbital relaxation with an explicit multi-configurational treatment.

physics.chem-ph↗

arXiv2026

C-Pol: Point charge perturbation scheme for mapping tensor moment surfaces

We present an efficient moment-based perturbation scheme for evaluating polarizability tensors of small molecules at a fraction of the computational cost of conventional energy-based approaches. Rather than applying explicit electric fields, the method perturbs the molecular charge density using strategically arranged external point charges, as in QM/MM simulations, and extracts polarizability tensors from finite differences of multipole moments extrapolated to the zero-perturbation limit. C-Pol is implemented as a backend-agnostic Python package requiring only that the host quantum chemistry code support external point-charge potentials. We demonstrate interfaces with three codes spanning complementary numerical approaches: GPAW (real-space grids), NWChem, and PySCF (atom-centered basis sets with complete basis set extrapolation). Validation against Gaussian 16 energy-based reference calculations across molecules representing all 19 commonly occurring point groups yields agreement within 3%, with the largest deviations confined to tensor components that are numerically small and contribute negligibly to the electrostatic potential. Finite-difference polarizabilities computed via C-Pol are further shown to agree with analytic coupled-perturbed Hartree-Fock values to within 0.02 a.u. across all tested basis sets, at comparable computational cost for medium-sized systems and with more favorable scaling for larger ones. The package provides a practical and transferable route to high-quality multipole polarizability data for force field development, polarizable embedding, and machine-learning training sets.

physics.chem-ph↗

arXiv2026

Polarizable Embedding QM/MM for Periodic Systems

A general polarizable embedded (PE) quantum mechanics/molecular mechanics scheme for periodic systems is presented, describing mutual polarization of the two subsystems. The QM system, described with density functional theory (DFT), is coupled to a single center multipole expansion (SCME) model, characterising H$_2$O molecules in the MM region. In SCME the H$_2$O molecules are ascribed anisotropic dipole and quadrupole polarizabilities and permanent multipoles up to and including the hexadecapole. Our embedding scheme illustrates a smooth and efficient convergence pattern of the periodic interaction potential by introducing a single and clustered multipole expansion points in the far-field. By choosing the near- and far-field expansion of the potential carefully the PE-QM/MM calculation matches the level of accuracy of a the QM calculation. In the short range, the electrostatic interaction between the QM and MM subsystems is damped with a real-space and pair-wise isotropic damping functions - resulting in a screened interaction and preventing over-polarization. In molecular dynamics simulations the two subsystems are separated with the elastic scattering assisted flexible inner region [Kirchhoff et. al. JCTC, 2021, 17, 9, 5863] - ensuring a smooth transition in the radial distribution at the boundary between the two subsystems.

physics.chem-ph↗

arXiv2026

Locally Scaled Self-Interaction Corrected Energy Functionals with Complex Optimal Orbitals

We present a fully variational locally scaled self-interaction corrected (SIC) energy functional using complex optimal orbitals. This represents an important milestone for fully variational SIC energy functionals, which have been shown to improve the prediction of the properties of atomic, molecular and solid state systems in general, in both ground and excited states. However, it depends on the system and property of the system whether it is beneficial to scale the SIC correction by a factor of one-half, which makes the application of SIC inconsistent. In the limit of a single electron the SIC exactly cancels the self interaction error, but overcorrects the error in regions of high density where there is large overlap between occupied orbitals. The newly implemented local scaling function, $z(\mathbf{r})$, which is based on an iso-orbital indicator derived from considering the kinetic energy density in the iso-electron and many electron case, and takes into account that the orbitals are complex, naturally scales the SIC correction from $0\leq z(\mathbf{r}) \leq 1$ in regions of high and low (isolated orbital) electron density. The locally scaled and fully variational SIC framework is general and applicable to atomic, molecular and solid-state systems.

physics.chem-ph↗

arXiv2025

Variational Calculations of the Excited States of the Charged NV-center in Diamond Using a Hybrid Functional

The excited electronic states involved in the optical cycle preparation of a pure spin state of the negatively charged NV-defect in diamond are calculated using the HSE06 hybrid density functional and variational optimization of the orbitals. This includes the energy of the excited triplet as well as the two lowest singlet states with respect to the ground triplet state. In addition to the vertical excitation, the effect of structural relaxation is also estimated using analytical atomic forces. The lowering of the energy in the triplet excited state and the resulting zero-phonon line triplet excitation energy are both within 0.1 eV of the experimental estimates. An analogous relaxation in the lower energy singlet state using spin purified atomic forces is estimated to be 0.06 eV. These results, obtained with a hybrid density functional, improve on previously published results using local and semi-local functionals, which are known to underestimate the band gap. The good agreement with experimental estimates demonstrates how time-independent variational calculations of excited states using density functionals can give accurate results and, thereby, provide a powerful screening tool for identifying other defect systems as candidates for quantum technologies.

cond-mat.mtrl-sci↗

arXiv2024

GPAW: An open Python package for electronic-structure calculations

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE) providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation (BSE), variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support of GPU acceleration has been achieved with minor modifications of the GPAW code thanks to the CuPy library. We end the review with an outlook describing some future plans for GPAW.

cond-mat.mtrl-sci↗

arXiv2023

On the challenge of obtaining an accurate solvation energy estimate in simulations of electrocatalysis

The effect of solvent on the free energy of reaction intermediates adsorbed on electrocatalyst surfaces can significantly change the thermochemical overpotential, but accurate calculations of this are challenging. Here, we present computational estimates of the solvation energy for reaction intermediates in oxygen reduction reaction (ORR) on a B-doped graphene (BG) model system where the overpotential is found to reduce by up to 0.6 V due to solvation. BG is experimentally reported to be an active ORR catalyst but recent computational estimates using state-of-the-art hybrid density functionals in the absence of solvation effects have indicated low activity. To test whether the inclusion of explicit solvation can bring the calculated activity estimates closer to the experimental reports, up to 4 layers of water molecules are included in the simulations reported here. The calculations are based on classical molecular dynamics and local minimization of energy using atomic forces evaluated from electron density functional theory. Data sets are obtained from regular and coarse-grained dynamics, as well as local minimization of structures resampled from dynamics simulations. The results differ greatly depending on the method used and the solvation energy estimates and are deemed untrustworthy. It is concluded that a significantly larger number of water molecules is required to obtain converged results for the solvation energy. As the present system includes up to 139 atoms, it already strains the limits of computational feasibility, so this points to the need for a hybrid simulation approach where efficient simulations of much larger number of solvent molecules is carried out using a lower level of theory while retaining the higher level of theory for the reacting molecules as well as their near neighbors and the catalyst.

physics.chem-ph↗

arXiv2022

Transferable Potential Function for Flexible H$_2$O Molecules Based on the Single Center Multipole Expansion

A potential function is presented for describing a system of flexible H$_2$O molecules based on the single center multipole expansion (SCME) of the electrostatic interaction. The model, referred to as SCME/f, includes the variation of the molecular quadrupole moment as well as the dipole moment with changes in bond length and angle so as to reproduce results of high level electronic structure calculations. The multipole expansion also includes fixed octupole and hexadecapole moments, as well as anisotropic dipole-dipole, dipole-quadrupole and quadrupole-quadrupole polarizability tensors. The model contains five adjustable parameters related to the repulsive interaction and damping functions in the electrostatic and dispersion interactions. Their values are adjusted to reproduce the lowest energy isomers of small clusters, (H$_2$O)$_n$ with $n=2-6$, as well as measured properties of the ice Ih crystal. Subsequent calculations of the energy difference between the various isomer configurations of the clusters show that SCME/f gives good agreement with results of electronic structure calculations and represents a significant improvement over the previously presented rigid SCME potential function. Analysis of the vibrational frequencies of the clusters and structural properties of ice Ih crystal show the importance of accurately describing the variation of the quadrupole moment with molecular structure.

physics.comp-ph↗

arXiv2021

Elastic Collision Based Dynamic Partitioning Scheme for Hybrid Simulations

The scattering-adapted flexible inner region ensemble separator (SAFIRES) is a partitioning scheme designed to divide a simulation cell into two regions to be treated with different computational methodologies. SAFIRES prevents particles from crossing between regions and resolves boundary events through elastic collisions of the particles mediated by the boundary, conserving energy and momenta. A multiple-time-step propagation algorithm is introduced where the time step is scaled automatically to identify the moment a collision occurs. If the length of the time step is kept constant, the new propagator reduces to a regular algorithm for Langevin dynamics, and to the velocity Verlet algorithm for classical dynamics if the friction coefficient is set to zero. SAFIRES constitutes the exact limit of the premise behind boundary-based methods such as FIRES, BEST, and BCC which take advantage of the indistinguishability of molecules on opposite sides of the separator. It gives correct average ensemble statistics despite the introduction of an ensemble separator. SAFIRES is tested in simulations where the molecules on the two sides are treated in the same way, for a Lennard-Jones (LJ) liquid and a LJ liquid in contact with a surface, as well as for liquid modelling simulations using the TIP4P force field. Simulations using SAFIRES are shown to reproduce the unconstrained reference simulations without significant deviations.

physics.chem-ph↗