UKRmol+: A suite for modelling electronic processes in molecules interacting with electrons, positrons and photons using the R-matrix method

UKRmol+ is a new implementation of the time-independent UK R-matrix electron–molecule scattering code. Key features of the implementation are the use of quantum chemistry codes such as Molpro to provide target molecular orbitals; the optional use of mixed Gaussian — B-spline basis functions to represent the continuum and improved configuration and Hamiltonian generation. The code is described, and examples covering electron collisions from a range of targets, positron collisions and photoionization are presented. The codes are freely available as a tarball from Zenodo

UKRmol-scripts: a Perl-based system for the automated operation of the photoionization and electron/positron scattering suite UKRmol+

UKRmol-scripts is a set of Perl scripts to automatically run the UKRmol+ codes, a complex software suite based on the R-matrix method to calculate fixed-nuclei photoionization and electron- and positron-scattering for polyatomic molecules. Starting with several basic parameters, the scripts operatively produce all necessary input files and run all codes for electronic structure and scattering calculations as well as gather the more frequently required outputs. The scripts provide a simple way to run such calculations for many molecular geometries concurrently and collect the resulting data for easier post-processing and visualization. We describe the structure of the scripts and the input parameters as well as provide examples for photoionization and electron and positron collisions with molecules. The codes are freely available from Zenodo

Ultrafast processes in N2 photoionization: implementacion of the XCHEM code

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de lectura: 27-09-2017Esta tesis tiene embargado el acceso al texto completo hasta el 27-03-2019This project studies the ultrafast photoionization (dissociative and non-dissociative) of molecular Nitrogen from a theoretical and computational point of view. Speci cally, we present the implementation of the XCHEM approach; a set of tools which allows the study, including electronic correlation, of photoionization going beyond simple benchmark systems. In particular we focussed on the successful description of autoionizing states; quasi-boundstates (coupled to continuum states) with a nite life time after which the system ionizes. Molecular Hydrogen was studied as a simple test case, allowing for comparison with results from well established methods. Subsequently the photoionization of molecular Nitrogen was studied in between the rst and third ionization threshold, i.e. where dissociation does not play a role. This is an important step establishing the XCHEM method's capability to handle complex multielectronic molecular systems. From a mathematical point of view the XCHEM method relies on a close coupling expansion of the electronic wavefunction, which is tted to the asymptotic behaviour seen in a Coulomb potential. The (ionic) scattering states are expanded in a novel hybrid Gaussian/B-Spline (GABS) basis set, whereas the (neutral) bound states are calculated using modi ed Quantum Chemistry Packages (QCP). As part of the development of this approach, this work focussed on the creation of original programs to be used in combination with modi ed and redesigned QCPs, allowing them to interface with the part of the XCHEM code dealing with scattering theory and involving GABS basis functions. The dissociatiove photoionization of molecular Nitrogen was also studied in collaboration with experimental e orts. In this experiment, Nitrogen was ionized using a single attosecond XUV pump pules and the subsequent dissociation dynamics was probed by a femtosecond IR pules (XUV pump/IR probe). The dependence of the kinetic energy of the fragments was recorded as a function of the delay between the pulses, giving rise to a clearly visible interference pattern. A theoretical description of this experiment, including large numbers of potential energy surfaces and taking into account non-adiabatic couplings, is proposed in this thesis and used to reproduce these results. From this model an interpretation of the observed experimental features is extracted.Este proyecto investiga desde punto de vista te orico y computacional la foto ionizaci on (disociativa y no disociativa) ultra r apida de la mol ecula de Nitr ogeno. Para simularla, presentamos la implementaci on del m etodo XCHEM : un conjunto de herramientas que permite estudiar la foto ionizaci on de sistemas complejos teniendo en cuenta correlaci on electr onica. En concreto, nos centramos en la descripci on de estados que experimentan autoionizaci on: estados casi ligados acoplados con estados del continuo. Dado que estos estados est an inmersos en el continuo, acaban decayendo a estados del cati on. Como primer sistema, hemos estudiado la mol ecula de Hidr ogeno lo que nos ha permitido comparar con resultados obtenidos con otros m etodos. Posteriormente hemos estudiado la foto ionizaci on de la mol ecula de Nitr ogeno entre el primer y el tercer umbral de ionizaci on. Eso constituye un paso importante, ya que establece la capacidad del m etodo XCHEM para describir sistemas complejos en mol eculas polielectr onicas. Desde el punto de vista matem atico el m etodo XCHEM esta basado en una expansi on close coupling, de la funci on de onda electr onica, que esta ajustada para describir el comportamiento asint otico observado en un potencial culombiano. Los estados del continuo (i onicos) se expanden en un nuevo conjunto de funciones de base que incluye una mezcla de funciones gausianas y B-Splines (GABS). Por otra parte, los estados ligados (neutros) se calculan con programas modi cadas de estructura electr onica (QCP). Como parte del desarrollo de este m etodo, en este trabajo se han programado una serie de programas originales que, en combinaci on con QCPs modi cados y redise~nados al efecto, permiten construir un interfaz entre estados ligados y estados del continuo. La foto ionizaci on disociativa de la mol ecula de Nitr ogeno se ha estudiado en colaboraci on con un grupo experimental. En el experimento, se emple o un pulso XUV de attosegundo para ionizar la mol ecula de Nitr ogeno y la din amica del proceso de disociaci on, provocada por este pulso, se sigui o con un pulso IR de femtosegundos a trav es de un esquema bombeo-sonda. Se midi o la dependencia de la energ a cin etica de los fragmentos molecular con el retraso entro los dos pulsos, dando lugar a un patr on de interferencia. En esta tesis, describimos este experimento usando un modelo te orico (que incluye cientos de super cies de energ a potencial con sus correspondientes acoplamientos no adiab aticos) que es capaz de reproducir los resultados experimentales. Adem as, profundizando en el modelo hemos sido capaces de entender la naturaleza del patr on de interferencia

Partial cross sections and interfering resonances in photoionization of molecular nitrogen

We present an in-depth theoretical study of N2 photoionization in the region between the second (2Πu) and third (2Σu+) ionization thresholds. In this region, the electronic continuum includes the Hopfield series of autoionizing states, corresponding to excitations to nsσd, ndσd, and ndπg molecular orbitals. Calculations have been performed by using the xchem code, which makes use of a Gaussian and B-spline hybrid basis in the framework of a close-coupling approach. We provide total and partial photoionization cross sections for all open channels, energy positions, and widths for the five lowest resonances of each series and, when resonances are well isolated from each other, Fano and Starace parameters. We also discuss how the coupling between the two series of overlapping resonances, nsσd and ndσd, affects their energies and autoionization widths. These results show the potential of the xchem method to describe resonant photoionization in moleculesThis work has been supported by the ERC Advanced Grant No. 290853 – XCHEM – within the Seventh Framework Program of the European Union, the ERC Proof-of-Concept Grant No. 780284 – Imaging-XChem – within the Horizon 2020 Framework Programme, and MINECO Project No. FIS2016-77889-R (AEI/FEDER, UE). L.A. acknowledges support from the TAMOP NSF through Grant No. 1607588, as well as UCF fundin

UKRmol+: A suite for modelling electronic processes in molecules interacting with electrons, positrons and photons using the R-matrix method

UKRmol+ is a new implementation of the time-independent UK R-matrix electron-molecule scattering code. Key features of the implementation are the use of quantum chemistry codes such as Molpro to provide target molecular orbitals; the optional use of mixed Gaussian – B-spline basis functions to represent the continuum and improved configuration and Hamiltonian generation. The code is described, and examples covering electron collisions from a range of targets, positron collisions and photoionization are presented. The codes are freely available as a tarball from Zenodo

Molecular Auger Decay Rates from Complex-Variable Coupled-Cluster Theory

The emission of an Auger electron is the predominant relaxation mechanism of core-vacant states in molecules composed of light nuclei. In this non-radiative decay process, one valence electron fills the core vacancy while a second valence electron is emitted into the ionization continuum. Because of this coupling to the continuum, core-vacant states represent electronic resonances that can be tackled with standard quantum-chemical methods only if they are approximated as bound states, meaning that Auger decay is neglected. Here, we present an approach to compute Auger decay rates of core-vacant states from coupled-cluster and equation-of-motion coupled-cluster wave functions combined with complex scaling of the Hamiltonian or, alternatively, complex-scaled basis functions. Through energy decomposition analysis, we illustrate how complex-scaled methods are capable of describing the coupling to the ionization continuum without the need to model the wave function of the Auger electron explicitly. In addition, we introduce in this work several approaches for the determination of partial decay widths and Auger branching ratios from complex-scaled coupled-cluster wave functions. We demonstrate the capabilities of our new approach by computations on core-ionized states of neon, water, dinitrogen, and benzene. Coupled-cluster and equation-of-motion coupled-cluster theory in the singles and doubles approximation both deliver excellent results for total decay widths, whereas we find partial widths more straightforward to evaluate with the former method. We also observe that the requirements towards the basis set are less arduous for Auger decay than for other types of resonances so that extensions to larger molecules are readily possible.Comment: 15 pages, 6 figures, 9 table

ASTRA: a Transition-Density-Matrix Approach to Molecular Ionization

We describe ASTRA (AttoSecond TRAnsitions), a new close-coupling approach to molecular ionization that uses many-body transition density matrices between ionic states with arbitrary spin and symmetry, in combination with hybrid integrals between Gaussian and numerical orbitals, to efficiently evaluate photoionization observables. Within the TDM approach, the evaluation of inter-channel coupling is exact and does not depend on the size of the configuration-interaction space of the ions. Thanks to these two crucial features, ASTRA opens the way to studying highly correlated and comparatively large targets at a manageable computational cost. Here, ASTRA is used to predict the parameters of bound and autoionizing states of the boron atom and of the N$_2$ molecule, as well as the total photoionization cross section of boron, N$_2$ and formaldehyde, H$_2$CO. Our results are in excellent agreement with available theoretical and experimental values from the literature. As a proof of principle of ASTRA's ability to tackle larger targets, we report preliminary results for the photoionization cross section of magnesium-porphyrin (MgH$_{12}$C$_{20}$N$_4$), a biologically relevant metallorganic complex with as many as 37 atoms.Comment: 30 pages, 8 figure