arXiv subjects

Edit Mátyus

Publications and source records attributed to Edit Mátyus.

At least 19 recordsLinked to original sources

arXiv2026

Vibrational infrared and Raman spectra of the methanol molecule with equivariant neural-network property surfaces

Electric dipole and polarizability surfaces are developed for the methanol (CH$_3$OH) molecule using ab initio electronic structure data, computed at the CCSD/aug-cc-pVTZ level of theory, and equivariant neural networks. These property surfaces are used to compute vibrational infrared and Raman intensities with variational vibrational energies and wave functions. The energies and wave functions, fully accounting for the large-amplitude motion and tunneling splitting states, are from continued variational vibrational computations, based on earlier work [Sunaga et al., J. Chem. Phys., 2025, 163, 064101], up to 3700 cm$^{-1}$ beyond the zero-point vibration, now reaching the O-H stretching fundamental. All vibrational fundamentals, combination and overtone bands are in excellent agreement with available (gas-phase) experimental data, with a 2.2 cm$^{-1}$ root-mean-squared deviation of the fundamentals from experiment. These developments constitute an important step towards a quantitative and comprehensive exact quantum dynamics model of the methanol molecule, and a linelist for astrophysical applications.

physics.chem-ph↗

arXiv2026

Rovibrational computations for the He$_2$ a $^3Σ_\mathrm{u}^+$ state including non-adiabatic, relativistic, and QED corrections

A potential energy curve (PEC) accurate to a fraction of 1 ppm ($1:10^6$) is computed for the $^3Σ_\mathrm{u}^+$ state of He$_2$ endowed with relativistic and QED corrections. The nuclear Schrödinger equation is solved on this PEC with diagonal Born-Oppenheimer and non-adiabatic mass corrections to obtain highly accurate rotational-vibrational levels. The computed rovibrational intervals and fine-structure splittings, spanning over several orders of magnitude in energy, are found to be in remarkable agreement with available high-resolution spectroscopy data.

physics.chem-ph↗

arXiv2026

Rovibrational computations for He$_2^+$ X $Σ_\mathrm{u}^+$ including non-adiabatic, relativistic and QED corrections

We report the potential energy curve, the diagonal Born-Oppenheimer, non-adiabatic mass, relativistic, and leading-order QED corrections for the ground electronic state of the helium dimer cation; the higher-order QED and finite-nuclear size effects are also estimated. The computations are carried out with an improved error control and over a much broader configuration range compared to earlier work [D. Ferenc, V. I. Korobov, and E. Mátyus, Phys. Rev. Lett. 125, 213001 (2020)]. As a result, all rovibrational bound states are reported with an estimated accuracy of 0.005 cm$^{-1}$.

physics.chem-ph↗

arXiv2025

Double-pair Coulomb and Breit photon correction to the correlated relativistic energy

The simplest, algebraic quantum-electrodynamical corrections, due to the double-negative energy subspace and instantaneous interactions, are computed to the no-pair energy of two-spin-1/2-fermion systems. Numerical results are reported for two-electron atoms with a clamped nucleus and positronium-like genuine two-particle systems. The Bethe-Salpeter equation provides the theoretical framework, and numerical methods have been developed for its equal-time time-slice. In practice, it requires solving a sixteen-component eigenvalue equation with a two-particle Dirac Hamiltonian, including the appropriate interaction. The double-pair corrections can either be included in the interaction part of the eigenvalue equation or treated as a perturbation to the no-pair Hamiltonian. The numerical results have an $α$ fine-structure constant dependence that is in excellent agreement with the known $α^3E_\mathrm{h}\$-order double-pair correction of non-relativistic quantum electrodynamics.

physics.chem-ph↗

arXiv2025

Spin-dependent terms of the Breit-Pauli Hamiltonian evaluated with an explicitly correlated Gaussian basis set for molecular computations

This work collects the spin-dependent leading-order relativistic and quantum-electrodynamical corrections for the electronic structure of atoms and molecules within the non-relativistic quantum electrodynamics framework. We report the computation of perturbative corrections using an explicitly correlated Gaussian basis set, which allows high-precision computations for few-electron systems. In addition to numerical tests for triplet Be, triplet H$_2$, and triplet H$_3^+$ states and comparison with no-pair Dirac-Coulomb-Breit Hamiltonian energies, numerical results are reported for electronically excited states of the helium dimer, He$_2$, for which the present implementation delivers high-precision magnetic coupling curves necessary for a quantitative understanding of the fine structure of its high-resolution rovibronic spectrum.

physics.chem-ph↗

arXiv2025

High-Precision Quantum Dynamics of He$_2$ over the b $^3Π_\mathrm{g}$-c $^3Σ_\mathrm{g}^+$ Electronic Subspace by including Non-adiabatic, Relativistic and QED Corrections and Couplings

Relativistic, quantum electrodynamics, as well as non-adiabatic corrections and couplings, are computed for the b $^3Π_\mathrm{g}$ and c $^3Σ_\mathrm{g}^+$ electronic states of the helium dimer. The underlying Born-Oppenheimer potential energy curves are converged to 1 ppm ($1:10^6$) relative precision using a variational explicitly correlated Gaussian approach. The quantum nuclear motion is computed over the b $^3Π_\mathrm{g}$-c $^3Σ_\mathrm{g}^+$ (and B $^1Π_\mathrm{g}$-C $^1Σ_\mathrm{g}^+$) 9-(12-)dimensional electronic-spin subspace coupled by non-adiabatic and relativistic (magnetic) interactions. The electron's anomalous magnetic moment is also included; its effect is expected to be visible in high-resolution experiments. The computed rovibronic energy intervals are in excellent agreement with available high-resolution spectroscopy data, including the rovibronic b $^3Π_\mathrm{g}$-state fine structure. Fine-structure splittings are also predicted for the c $^3Σ_\mathrm{g}^+$ levels, which have not been fully resolved experimentally, yet.

physics.chem-ph↗

arXiv2025

Rovibrational computation of H$_3^+$ with permutationally invariant Pekeris coordinates

The Pekeris coordinates provide a permutationally invariant set of coordinates for H$_3^+$. They are defined as linear combinations of the three internuclear distances that automatically fulfil the triangle inequality for all non-negative coordinate values. In this work, we test three discrete variable representations (DVR) for tightly converging the rovibrational energies up to and beyond the barrier to linearity using the Pekeris coordinates. The best performing representation is a cot-DVR-type approach adapted to the Pekeris problem. The two- and three-proton near coalescence region, which is also part of the direct product Pekeris grid but dynamically not relevant, is avoided by coordinate mapping and regulator functions.

physics.chem-ph↗

arXiv2024

One-particle operator representation over two-particle basis sets for relativistic QED computations

This work is concerned with two-spin-1/2-fermion relativistic quantum mechanics, and it is about the construction of one-particle projectors using an inherently two(many)-particle, `explicitly correlated' basis representation, necessary for good numerical convergence of the interaction energy. It is demonstrated that a faithful representation of the one-particle operators, which appear in intermediate but essential computational steps, can be constructed over a many-particle basis set by accounting for the full Hilbert space beyond the physically relevant anti-symmetric subspace. Applications of this development can be foreseen for the computation of quantum-electrodynamics corrections for a correlated relativistic reference state and high-precision relativistic computations of medium-to-high-$Z$ helium-like systems, for which other two-particle projection techniques are unreliable.

physics.chem-ph↗

arXiv2024

Bound-state relativistic quantum electrodynamics: a perspective for precision physics with atoms and molecules

Precision physics aims to use atoms and molecules to test and develop the fundamental theory of matter, possibly beyond the Standard Model. Most of the atomic and molecular phenomena are described by the QED (quantum electrodynamics) sector of the Standard Model. Do we have the computational tools, algorithms, and practical equations for the most possible complete computation of atoms and molecules within the QED sector? What is the fundamental equation to start with? Is it still Schrödinger's wave equation for molecular matter, or is there anything beyond that? This paper provides a concise overview of the relativistic QED framework and recent numerical developments targeting precision physics and spectroscopy applications with common features with the robust and successful relativistic quantum chemistry methodology.

physics.chem-ph↗

arXiv2024

Regularized relativistic corrections for polyelectronic and polyatomic systems with explicitly correlated Gaussians

Drachmann's regularization approach is implemented for floating explicitly correlated Gaussians (fECGs) and molecular systems. Earlier applications of drachmannized relativistic corrections for molecular systems were hindered due to the unknown analytic matrix elements of $1/r_{ix}1/r_{jy}$-type operators with fECGs. In the present work, one of the $1/r$ factors is approximated by a linear combination of Gaussians, which results in calculable integrals. The numerical approach is found to be precise and robust over a range of molecular systems and nuclear configurations, and thus, it opens the route towards an automated evaluation of high-precision relativistic corrections over potential energy surfaces of polyatomic systems. Furthermore, the newly developed integration approach makes it possible to construct the matrix representation of the square of the electronic Hamiltonian relevant for energy lower-bound as well as time-dependent computations of molecular systems with a flexible and high-precision fECG basis representation.

physics.chem-ph↗

arXiv2024

QED corrections to the correlated relativistic energy: one-photon processes

This work is a collection of initial calculations and formal considerations within the Salpeter-Sucher exact equal-time relativistic quantum electrodynamics framework. The calculations are carried out as preparation for the computation of pair, retardation, and radiative corrections to the relativistic energy of correlated two-spin-1/2-fermion systems. In this work, particular attention is paid to the retardation and the `one-loop' self-energy corrections, which are known to be among the largest corrections to the correlated relativistic energy. The theoretical development is supplemented with identifying formal connections to the non-relativistic quantum electrodynamics framework, which is based on a correlated but non-relativistic reference, as well as to the `$1/Z$ approach', which is built on a relativistic but independent-particle zeroth order. The two complementary directions currently provide the theoretical framework for light atomic-molecular precision spectroscopy and heavy-atom phenomena. The present theoretical efforts pave the way for relativistic QED corrections to (explicitly) correlated relativistic computations.

physics.chem-ph↗

arXiv2024

Methane dimer rovibrational states and Raman transition moments

Benchmark-quality rovibrational data are reported for the methane dimer from variational nuclear motion computations using an ab initio intermolecular potential energy surface reported by [M. P. Metz et al., Phys. Chem. Chem. Phys., 2019, 21, 13504-13525]. A simple polarizability model is used to compute Raman transition moments that may be relevant for future direct observation of the intermolecular dynamics. Non-negligible $ΔK\neq 0$ transition moments arise in this symmetric top system due to strong rovibrational couplings.

physics.chem-ph↗

arXiv2023

Vibrational infrared and Raman spectrum of HCOOH from variational computations

All vibrational energies of the (trans-, cis-, delocalized-) formic acid molecule are converged up to 4500 cm$^{-1}$ beyond the zero-point vibrational energy with the GENIUSH-Smolyak variational approach and using an ab initio potential energy surface [D. P. Tew and W. Mizukami, J. Phys. Chem. A, 120, 9815-9828 (2016)]. Full-dimensional dipole and polarizability surfaces are fitted to points computed at the CCSD/aug-cc-pVTZ level of theory. Then, body-fixed vibrational dipole and polarizability transition moments are evaluated and used to simulate jet-cooled infrared and Raman spectra of HCOOH. The benchmark-quality vibrational energy, transition moment, and wave function list will be used in further work in comparison with vibrational experiments, and in further rovibrational computations.

physics.chem-ph↗

arXiv2023

Pre-Born-Oppenheimer Dirac-Coulomb-Breit computations for two-body systems

The sixteen-component, no-pair Dirac--Coulomb--Breit equation, derived from the Bethe--Salpeter equation, is solved in a variational procedure using Gaussian-type basis functions for the example of positronium, muonium, hydrogen atom, and muonic hydrogen. The $α$ fine-structure-constant dependence of the variational energies, through fitting a function of $α^n$ and $α^n\text{ln}α$ terms, shows excellent agreement with the relevant energy expressions of the (perturbative) non-relativistic QED framework, and thereby, establishes a solid reference for the development of a computational relativistic QED approach.

quant-ph↗

arXiv2023

The Bethe-Salpeter QED wave equation for bound-state computations of atoms and molecules

Interactions in atomic and molecular systems are dominated by electromagnetic forces and the theoretical framework must be in the quantum regime. The physical theory for the combination of quantum mechanics and electromagnetism, quantum electrodynamics has been established by the mid-twentieth century, primarily as a scattering theory. To describe atoms and molecules, it is important to consider bound states. In the non-relativistic quantum mechanics framework, bound states can be efficiently computed using robust and general methodologies with systematic approximations developed for solving wave equations. With the sight of the development of a computational quantum electrodynamics framework for atomic and molecular matter, the field theoretic Bethe-Salpeter wave equation expressed in space-time coordinates, its exact equal-time variant and emergence of a relativistic wave equation is reviewed. A computational framework, with initial applications and future challenges in relation with precision spectroscopy, is also highlighted.

physics.chem-ph↗

arXiv2023

Relativistic two-electron atomic and molecular energies using $LS$ coupling and double groups: role of the triplet contributions to singlet states

The triplet contribution is computed to the 1 and 2 $^1S^\text{e}_0$ states of the He atom, to the $1\ ^1S^\text{e}_0$ state of the Li$^+$ and Be${^{2+}}$ ions, and to the $X\ ^1Σ_\text{g}^+$ ground state of the H$_2$ molecule by extensive use of double-group symmetry (equivalent to $LS$ coupling for the atomic systems) during the course of the variational solution of the no-pair Dirac-Coulomb-Breit wave equation. The no-pair Dirac-Coulomb-Breit energies are converged within a sub-parts-per-billion relative precision using an explicitly correlated Gaussian basis optimized to the non-relativistic energies. The $α$ fine-structure constant dependence of the triplet sector contribution to the variational energy is $α^4E_\text{h}$ at leading order, in agreement with the formal perturbation theory result available from the literature.

physics.atom-ph↗

arXiv2023

Evaluation of the Bethe logarithm: from atom to chemical reaction

A general computational scheme for the (non-relativistic) Bethe logarithm is developed opening the route to `routine' evaluation of the leading-order quantum electrodynamics correction (QED) relevant for spectroscopic applications for small polyatomic and polyelectronic molecular systems. The implementation relies on Schwartz' method and minimization of a Hylleraas functional. In relation with electronically excited states, a projection technique is considered, which ensures positive definiteness of the functional over the entire parameter (photon momentum) range. Using this implementation, the Bethe logarithm is converged to a relative precision better than 1:10$^3$ for selected electronic states of the two-electron H$_2$ and H$_3^+$, and the three-electron He$_2^+$ and H+H$_2$ molecular systems. The present work focuses at nuclear configurations near the local minimum of the potential energy surface, but the computations can be repeated also for other structures.

physics.chem-ph↗

arXiv2023

Pre-Born-Oppenheimer energies, leading-order relativistic and QED corrections for electronically excited states of molecular hydrogen

For rovibronic states corresponding to the $B$ and $B'\ ^1Σ_\text{u}^+$ electronic states of the hydrogen molecule, the pre-Born--Oppenheimer (four-particle) non-relativistic energy is converged to a 1-3 parts-per-billion relative precision. The four-particle non-relativistic energy is appended with leading-order relativistic, leading- and estimated higher-order quantum-electrodynamics corrections. The resulting term values referenced to the rovibronic ground state are obtained in an excellent agreement with the experimental results. Further results are reported and discussed for other rovibronic states assignable to the $C\ ^1Π_\text{u}$ and the $EF,GK,$ and $HH\ ^1Σ_\text{g}^+$ electronic states.

physics.chem-ph↗