arXiv subjects

Hakon Volkmann

Publications and source records attributed to Hakon Volkmann.

4 recordsLinked to original sources

arXiv2026

Ab initio multichannel calculation of the Bethe surface and Compton defects for molecular hydrogen

A correlated multichannel computation of the generalized oscillator-strength density for electrons scattering off randomly oriented molecular hydrogen at the internuclear separation of 1.4 $a_0$ is being presented. The numerical procedure features a novel free-boundary method based on configuration interaction of ionic orbitals expanded in B splines using prolate-spheroidal coordinates. Satisfactory agreement between the present and both experimental as well as other theoretical results has been found, thus providing a view on the Bethe surface for energy transfers up to of 558 eV. The systematic and robust convergence behavior of the method further allowed for assessing the validity of the binary-encounter approximation, providing a confirmation of experimentally determined Compton defects for H$_2$ using a single consistent theoretical description. Overall, the present findings are expected to be helpful for calculating highly accurate energy-loss spectra for fast electrons passing through and ionizing molecular hydrogen gas or its isotopologues.

quant-ph↗

arXiv2026

Accurate theoretical methods for photoionization of H$_2$ molecules

Single-photon ionization cross sections of molecular hydrogen in the electric dipole limit are numerically computed by employing three independent methods. Both time-dependent and -independent approaches within the clamped-nuclei approximation at equilibrium internuclear distance are used, featuring the explicit time-propagation of the time-dependent Schrödinger equation and newly developed multi-channel configuration-interaction free-boundary as well as complex-scaling methods using an explicitly correlated geminal basis set. The found results show convincing mutual agreement despite their entirely different fundamental formulations. They further highlight the challenges in bringing together different theoretical predictions from literature with the experimental data at high photon energies. Overall, the novel CI-based approach demonstrates fast and controllable convergence while being able to provide full channel-resolved information, indicating the need for more accurate experimental data.

quant-ph↗

arXiv2023

A qubit-ADAPT Implementation for H$_2$ Molecules using an Explicitly Correlated Basis

With the recent advances in the development of devices capable of performing quantum computations, a growing interest in finding near-term applications has emerged in many areas of science. In the era of non-fault tolerant quantum devices, algorithms that only require comparably short circuits accompanied by high repetition rates are considered to be a promising approach for assisting classical machines with finding solution on computationally hard problems. The ADAPT approach previously introduced in Nat. Commun. 10, 3007 (2019) extends the class of variational quantum eigensolver (VQE) algorithms with dynamically growing ansätze in order to find approximations to ground and excited state energies of molecules. In this work, the ADAPT algorithm has been combined with a first-quantized formulation for the hydrogen molecule in the Born-Oppenheimer approximation, employing the explicitly correlated basis functions introduced in J. Chem. Phys. 43, 2429 (1965). By the virtue of their explicit electronic correlation properties, it is shown in classically performed simulations that relatively short circuits yield chemical accuracy ($< 1.6$ mHa) for ground and excited state potential curves that can compete with second quantized approaches such as Unitary Coupled Cluster.

quant-ph↗

arXiv2021

Potentials, exchange, and correlation in attosecond photoemission delays

We investigate attosecond time delays in the emission of photoelectrons using a hierarchy of models of the $CO_2$ molecule including the strong field approximation, Coulomb-scattering, short-range parts of the molecular potential, Hartree and Hartree-Fock descriptions. In addition, we present an {\it ab initio} calculation based on quantum-chemical structure in combination with strong-field techniques, which fully includes multi-electron exchange and correlation. Every single of these model constituents is found to modify delays on the scale of 10 as or more, with exchange and correlation having the most pronounced effect.

physics.atom-ph↗