### Browsing by Author "Jonsson, Hannes"

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Item Atomic scale simulations of heterogeneous electrocatalysis: recent advances(2017) Skúlason, Egill; Jonsson, Hannes; Department of Applied Physics; University of IcelandThe methodology for atomic scale calculations of electrocatalysis in order to identify mechanisms and estimate reaction rates is reviewed. These include: (1) the application of an external electrical field or potential in density functional theory calculations, (2) the thermochemical model for estimating the onset potential of an electrochemical reaction, and (3) calculations of transition paths in atomic scale models of the electrical double layer. Hydrogen evolution reaction, oxygen reduction reaction as well as CO2 and N-2 electrochemical reduction to form methane and ammonia are taken as examples. Calculations of reaction rates based on the estimation of the activation energy of elementary steps from minimum energy paths and transition state theory have been shown to provide accurate estimates of rates even for complex reactions and competing reaction mechanisms. There is room, however, for further improvements and some of those are also mentioned at the end of this mini-review.Item Calculations of Product Selectivity in Electrochemical CO2 Reduction(2018-06) Hussain, Javed; Jonsson, Hannes; Skulason, Egill; Department of Applied Physics; University of IcelandCO2 can be reduced electrochemically to form valuable chemicals such as hydrocarbons and alcohols using copper electrodes, whereas the other metal electrodes tested so far mainly form CO or formate, or only the side product, H-2. Accurate modeling of electrochemical reaction rates including the complex environment of an electrical double layer in the presence of an applied electrical potential is challenging. We show here that calculated rates, obtained using a combination of density functional and rate theory, are in close agreement with available experimental data on the formation of the various products on several metal electrodes and over a range in applied potential, thus demonstrating the applicability of the theoretical methodology. The results explain why copper electrodes give a significant yield of hydrocarbons and alcohols, and why methane, ethylene, and ethanol are formed in electroreduction rather than methanol, which is the main product when H-2 gas reacts with CO2 on copper catalyst. The insight obtained from the calculations is used to develop criteria for identifying new and improved catalysts for electrochemical CO2 reduction.Item Calculations of the onset temperature for tunneling in multispin systems(ST PETERSBURG NATL RESEARCH UNIV INFORMATION TECHNOLOGIES, MECH & OPTICS, 2017) Vlasov, S. M.; Bessarab, P.F.; Uzdin, V.M.; Jonsson, Hannes; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Department of Applied PhysicsTransitions between magnetic states of a system coupled to a heat bath can occur by exceeding the energy barrier, but as temperature is lowered quantum mechanical tunneling through the barrier becomes the dominant transition mechanism. A method is presented for estimating the onset temperature for tunneling in a system with an arbitrary number of spins using the second derivatives of the energy with respect to the orientation of the magnetic vectors at the first order saddle point on the energy surface characterizing the over-the-barrier mechanism. An application to a monomer and a dimer of molecular magnets containing a Mn 4 group is presented and the result found to be in excellent agreement with reported experimental measurements.Item Effect of H adsorption on the magnetic properties of an Fe island on a W(110) surface(American Physical Society, 2019-11-25) Melander, Marko; Jonsson, Hannes; University of Jyväskylä; Multiscale Statistical and Quantum Physics; Department of Applied PhysicsLow-dimensional materials, such as ultrathin films, nanoislands, and wires, are actively being studied due to their interesting magnetic properties and possible technological applications for example in high density data storage. Results of density functional theory calculations within the generalized gradient approximation of an Fe nanoisland on a W(110) surface are presented here with particular focus on the effect of hydrogen adsorption on magnetic properties. The adsorption is found to strongly decrease the magnetic moment of the Fe atoms the H atoms are bound to, down to less than a half in some cases as compared with the clean Fe island. This is an important consideration since hydrogen can relatively easily be introduced and removed from the system, thus providing a way of tuning magnetic properties, and it can also be unintentionally present even under ultrahigh vacuum conditions, especially at low temperature.Item Fully self-consistent calculations of magnetic structure within non-collinear Alexander-Anderson model(ST PETERSBURG NATL RESEARCH UNIV INFORMATION TECHNOLOGIES, MECH & OPTICS, 2020-02) Ivanov, A.; Bessarab, P. F.; Jonsson, Hannes; Uzdin, V. M.; University of Iceland; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Multiscale Statistical and Quantum Physics; Department of Applied PhysicsAn implementation of the non-collinear Alexander-Anderson model for itinerant electrons in magnetic systems is presented where self-consistency is reached for specified directions of the magnetic moments. This is achieved by means of Lagrange multipliers and a variational principle for determining the transverse and longitudinal components of the magnetic moments as well as the average number of d-electrons using direct optimisation. Various optimisation algorithms are compared and the limited memory Broyden-Fletcher-Goldfarb-Shanno algorithm is found to give the best performance. An application to antiferromagnetic Cr crystal is presented where spin-dynamics and curvature of the energy surface are calculated to compare results obtained with and without the constraints on the orientation of the magnetic moments.Item Magnetic skyrmion annihilation by quantum mechanical tunneling(IOP Publishing Ltd., 2020-08) Vlasov, Sergei M.; Bessarab, Pavel F.; Lobanov, Igor S.; Potkina, Mariia N.; Uzdin, Valery M.; Jonsson, Hannes; Multiscale Statistical and Quantum Physics; Department of Applied PhysicsMagnetic skyrmions are nano-scale magnetic states that could be used in various spintronics devices. A central issue is the mechanism and rate of various possible annihilation processes and the lifetime of metastable skyrmions. While most studies have focused on classical over-the-barrier mechanism for annihilation, it is also possible that quantum mechanical tunneling through the energy barrier takes place. Calculations of the lifetime of magnetic skyrmions in a two-dimensional lattice are presented and the rate of tunneling compared with the classical annihilation rate. A remarkably strong variation in the onset temperature for tunneling and the lifetime of the skyrmion is found as a function of the values of parameters in the extended Heisenberg Hamiltonian, i.e. the out-of-plane anisotropy, Dzyaloshinskii-Moriya interaction and applied magnetic field. Materials parameters and conditions are identified where the onset of tunneling could be observed on a laboratory time scale. In particular, it is predictedthat skyrmion tunneling could be observed in the PdFe/Ir(111) system when an external magnetic field on the order of 6Tis applied.Item Minimum mode saddle point searches using Gaussian process regression with inverse-distance covariance function(AMERICAN CHEMICAL SOCIETY, 2020-01-14) Koistinen, Olli-Pekka; Ásgeirsson, Vilhjálmur; Vehtari, Aki; Jonsson, Hannes; Probabilistic Machine Learning; University of Iceland; Multiscale Statistical and Quantum Physics; Department of Computer Science; Department of Applied PhysicsThe minimum mode following method can be used to find saddle points on an energy surface by following a direction guided by the lowest curvature mode. Such calculations are often started close to a minimum on the energy surface to find out which transitions can occur from an initial state of the system, but it is also common to start from the vicinity of a first order saddle point making use of an initial guess based on intuition or more approximate calculations. In systems where accurate evaluations of the energy and its gradient are computationally intensive, it is important to exploit the information of the previous evaluations to enhance the performance. Here, we show that the number of evaluations required for convergence to the saddle point can be significantly reduced by making use of an approximate energy surface obtained by a Gaussian process model based on inverse inter-atomic distances, evaluating accurate energy and gradient at the saddle point of the approximate surface and then correcting the model based on the new information. The performance of the method is tested with start points chosen randomly in the vicinity of saddle points for dissociative adsorption of an H2 molecule on the Cu(110) surface and three gas phase chemical reactions.Item Nudged elastic band calculations accelerated with Gaussian process regression(2017-10-21) Koistinen, Olli-Pekka; Dagbjartsdóttir, Freyja B.; Ásgeirsson, Vilhjálmur; Vehtari, Aki; Jonsson, Hannes; Department of Computer Science; Department of Applied Physics; Professorship Vehtari Aki; Helsinki Institute for Information Technology (HIIT); Probabilistic Machine Learning; Multiscale Statistical and Quantum Physics; University of IcelandMinimum energy paths for transitions such as atomic and/or spin rearrangements in thermalized systems are the transition paths of largest statistical weight. Such paths are frequently calculated using the nudged elastic band method, where an initial path is iteratively shifted to the nearest minimum energy path. The computational effort can be large, especially when ab initio or electron density functional calculations are used to evaluate the energy and atomic forces. Here, we show how the number of such evaluations can be reduced by an order of magnitude using a Gaussian process regression approach where an approximate energy surface is generated and refined in each iteration. When the goal is to evaluate the transition rate within harmonic transition state theory, the evaluation of the Hessian matrix at the initial and final state minima can be carried out beforehand and used as input in the minimum energy path calculation, thereby improving stability and reducing the number of iterations needed for convergence. A Gaussian process model also provides an uncertainty estimate for the approximate energy surface, and this can be used to focus the calculations on the lesser-known part of the path, thereby reducing the number of needed energy and force evaluations to a half in the present calculations. The methodology is illustrated using the two-dimensional Müller-Brown potential surface and performance assessed on an established benchmark involving 13 rearrangement transitions of a heptamer island on a solid surface.Item Nudged elastic band calculations accelerated with Gaussian process regression based on inverse interatomic distances(AMERICAN CHEMICAL SOCIETY, 2019-12-10) Koistinen, Olli-Pekka; Ásgeirsson, Vilhjálmur; Vehtari, Aki; Jonsson, Hannes; Department of Computer Science; Department of Applied Physics; Probabilistic Machine Learning; Professorship Vehtari Aki; Multiscale Statistical and Quantum Physics; University of IcelandCalculations of minimum energy paths for atomic rearrangements using the nudged elastic band method can be accelerated with Gaussian process regression to reduce the number of energy and atomic force evaluations needed for convergence. Problems can arise, however, when configurations with large forces due to short distance between atoms are included in the data set. Here, a significant improvement to the Gaussian process regression approach is obtained by basing the difference measure between two atomic configurations in the covariance function on the inverted inter-atomic distances and by adding a new early stopping criterion for the path relaxation phase. This greatly improves the performance of the method in two applications where the original formulation does not work well: a dissociative adsorption of an H2 molecule on a Cu(110) surface and a diffusion hop of an H2O molecule on an ice Ih(0001) surface. Also, the revised method works better in the previously analyzed benchmark application to rearrangement transitions of a heptamer island on a surface, requiring fewer energy and force evaluations for convergence to the minimum energy path.Item Polymer escape from a confining potential(AIP Publishing, 2014) Mökkönen, Harri; Ikonen, Timo; Jonsson, Hannes; Ala-Nissilä, Tapio; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe rate of escape of polymers from a two-dimensionally confining potential well has been evaluated using self-avoiding as well as ideal chain representations of varying length, up to 80 beads. Long timescale Langevin trajectories were calculated using the path integral hyperdynamics method to evaluate the escape rate. A minimum is found in the rate for self-avoiding polymers of intermediate length while the escape rate decreases monotonically with polymer length for ideal polymers. The increase in the rate for long, self-avoiding polymers is ascribed to crowding in the potential well which reduces the free energy escape barrier. An effective potential curve obtained using the centroid as an independent variable was evaluated by thermodynamic averaging and Kramers rate theory then applied to estimate the escape rate. While the qualitative features are well reproduced by this approach, it significantly overestimates the rate, especially for the longer polymers. The reason for this is illustrated by constructing a two-dimensional effective energy surface using the radius of gyration as well as the centroid as controlled variables. This shows that the description of a transition state dividing surface using only the centroid fails to confine the system to the region corresponding to the free energy barrier and this problem becomes more pronounced the longer the polymer is. A proper definition of a transition state for polymer escape needs to take into account the shape as well as the location of the polymer.Item Skyrmions in antiferromagnets(American Institute of Physics, 2020-06-07) Potkina, Maria N.; Lobanov, Igor S.; Jonsson, Hannes; Uzdin, Valery M.; ITMO University; Multiscale Statistical and Quantum Physics; Department of Applied PhysicsCalculations of skyrmions in antiferromagnets (AFMs) are presented, and their properties compared with skyrmions in corresponding ferromagnets (FMs). The rates of skyrmion collapse and escape through the boundary of a track, as well as the binding to and collapse at a non-magnetic impurity, are calculated as a function of an applied magnetic field. The activation energy for skyrmion annihilation is the same in AFMs and corresponding FMs in the absence of an applied magnetic field. The pre-exponential factor in the Arrhenius rate law is, however, different because skyrmion dynamics is different in the two systems. An applied magnetic field has opposite effects on skyrmions in the two types of materials. In AFMs, the rate of collapse of skyrmions as well as the rate of escape through the edge of a magnetic strip decreases slightly with increasing field, while these rates increase strongly for a skyrmion in the corresponding FMs when the field is directed antiparallel to the magnetization in the center of the skyrmion. A non-magnetic impurity is less likely to trap a skyrmion in AFMs, especially in the presence of a magnetic field. This, together with the established fact that a spin polarized current moves skyrmions in AFMs in the direction of the current, while in FMs skyrmions move at an angle to the current, demonstrates that skyrmions in AFMs have several advantageous properties over skyrmions in FMs for memory and spintronic devices.Item Transition State Theory Approach to Polymer Escape from a One Dimensional Potential Well(2015) Mökkönen, Harri; Ikonen, T.; Ala-Nissila, Tapio; Jonsson, Hannes; Department of Applied PhysicsThe rate of escape of an ideal bead-spring polymer in a symmetric double-well potential is calculated using transition state theory (TST) and the results compared with direct dynamical simulations. The minimum energy path of the transitions becomes flat and the dynamics diffusive for long polymers making the Kramers-Langer estimate poor. However, TST with dynamical corrections based on short time trajectories started at the transition state gives rate constant estimates that agree within a factor of two with the molecular dynamics simulations over a wide range of bead coupling constants and polymer lengths. The computational effort required by the TST approach does not depend on the escape rate and is much smaller than that required by molecular dynamics simulations.