Hannes Jónsson

Calculations based on density functional theory and statistical mechanics reveal how Fe site preference in olivine is altered at interfaces.

Rydberg excited states of molecules pose a challenge for electronic structure calculations because of their highly diffuse electron distribution. Even large and elaborate atomic basis sets tend to underrepresent the long-range tail, overly confining the Rydberg state. An approach is presented here where the molecular orbitals are variationally optimized for the excited state using a plane wave basis set in a Hartree-Fock calculation, followed by a configuration interaction calculation. The use of excited state optimized orbitals greatly enhances the convergence of the many-body calculation, as illustrated by a full configuration interaction calculation of the 2s Rydberg state of H₂. A neural-network-based selective configuration interaction approach is then applied to calculations of 3s and 3p states of H₂O and NH₃. The obtained values of excitation energy are in close agreement with experimental measurements as well as previous many-body calculations where sufficiently diffuse atomic basis sets were used. Calculations using atomic basis sets lacking extra diffuse functions, such as aug-cc-pVTZ, give significantly higher estimates due to confinement of the Rydberg states.

Electrolyte solutions at high concentration are indispensable and yet poorly understood. In particular, the extent of speciation─the formation of complexes composed of multiple species─in concentrated ionic solutions is very challenging to obtain theoretically and experimentally, but can have a strong effect on solution properties. The literature is rife with contradictory estimates of speciation from experiments. We find that speciation affects transport properties and is therefore a prerequisite to accurately model concentrated solutions. We turn this to our advantage by showing that the viscosity can be used to determine the extent of complexation in concentrated aqueous solutions. Results of simulations as well as experimental measurements are presented. The atomistic Madrid-2019 force field is extended to model FeCl₂. Solutions of FeCl₂ and MgCl₂ are compared, and the observed difference in viscosity is explained by more complexation in the former, a conclusion supported by recently reported X-ray absorption and neutron scattering experiments.

Gaussian process (GP) regression provides a strategy for accelerating saddle point searches on high‐dimensional energy surfaces by reducing the number of times the energy and its derivatives with respect to atomic coordinates need to be evaluated. The computational overhead in the hyperparameter optimization can, however, be large and make the approach inefficient. Failures can also occur if the search ventures too far into regions that are not represented well enough by the GP model. Here, these challenges are resolved by using geometry‐aware optimal transport measures and an active pruning strategy using a summation over Wasserstein‐1 distances for each atom‐type in farthest‐point sampling, selecting a fixed‐size subset of geometrically diverse configurations to avoid rapidly increasing cost of GP updates as more observations are made. Stability is enhanced by a permutation‐invariant metric that provides a reliable trust radius for early‐stopping and a logarithmic barrier penalty for the growth of the signal variance. These physically motivated algorithmic changes prove their efficacy by reducing to less than a half the mean computational time on a set of 238 challenging configurations from a previously published data set of chemical reactions. With these improvements, the GP approach is established as a robust and scalable algorithm for accelerating saddle point searches when the evaluation of the energy and atomic forces requires significant computational effort.

The effect of modifying substituents in the rotor group of five second generation molecular motors is estimated by theoretical calculations. The rotational speed is estimated by calculating the rate limiting step, the thermal helix inversion, and the competing backward transition using harmonic transition state theory with energy and atomic forces obtained from density functional theory. First, a methyl group at the stereogenic center is replaced with a tert-butyl (tBu) group, and the rotational speed is found to increase due to reduced lifetime of the metastable state. For two of the rotors, comparison can be made with experimental measurements, and the calculated half-life is in close agreement. Second, the effect of substituting the nine hydrogen atoms in the tBu group with fluorine atoms is studied, and this is found to increase the rotational rate further without significantly altering the molecular structure. The excitation wavelength of both the stable and metastable states is calculated, and the separation of the absorption peaks is found to increase by the tBu substitution and even more so by the fluorinated tBu substitution, up to 40 nm. These findings can help to develop a strategy for designing molecular motors with a rotational speed that best fits a given application.