New computational approaches for inelastic x-ray scattering

Inelastic x-ray scattering spectroscopy is a versatile experimental technique for probing the electronic structure of materials. It provides a wealth of information on the sample's atomic-scale structure, but extracting this information from the experimental data can be challenging because there is no direct relation between the structure and the measured spectrum. Theoretical calculations can bridge this gap by explaining the structural origins of the spectral features. Reliable methods for modeling inelastic x-ray scattering require accurate electronic structure calculations. This work presents the development and implementation of new schemes for modeling the inelastic scattering of x-rays from non-periodic systems. The methods are based on density functional theory and are applicable for a wide variety of molecular materials. Applications are presented in this work for amorphous silicon monoxide and several gas phase systems. Valuable new information on their structure and properties could be extracted with the combination of experimental and computational methods.Materiaalien atomitason järjestäytymistä ja elektronirakennetta tutkitaan nykyään tiiviisti erilaisia kokeellisia sekä teoreettisia menetelmiä hyödyntäen. Nämä mikroskooppiset ominaisuudet määrittävät lopulta myös aineiden näkyvät ja toiminnalliset ominaisuudet, ja niiden tarkka ymmärtäminen voi mahdollistaa uusien ja halutulla tavalla käyttäytyvien materiaalien kehittämisen. Epäelastinen röntgensirontaspektroskopia on kokeellinen menetelmä aineen atomimittakaavan ominaisuuksien tutkimiseksi. Menetelmällä saadaan yksityiskohtaista tietoa aineen rakenteesta kun kokeellinen mittaustulos yhdistetään aineen atomitason ominaisuuksiin. Valitettavasti näiden välillä ei ole suoraa kytkentää. Tarkkojen laskennallisten menetelmien avulla yhdistäminen kuitenkin onnistuu kun mittaustulosta simuloidaan rakennemallista lähtien. Tähän tarkoitukseen soveltuvat menetelmät vaativat raskasta laskentaa ja aineen elektronien vuorovaikutuksen sekä elektronirakenteen viritystiloihin liittyvien kvanttimekaanisten ilmiöiden mallintamista. Tässä työssä kehitettiin uusia laskennallisia menetelmiä epäelastisen röntgensironnan mallintamiseen molekyylimateriaaleista. Menetelmät perustuvat elektronirakenteen ja elektroniviritysten mallintamiseen tiheysfunktionaaliteoriaa käyttäen. Vertaamalla mallinnettua ja kokeellisesti havaittua epäelastista sirontaspektriä voidaan tehdä johtopäätöksiä tutkittujen materiaalien ominaisuuksista. Kehitettyjä menetelmiä sovellettiin mm. optoelektroniikassa hyödynnettävien piioksidimateriaalien rakenteen selvittämiseen sekä eräiden kaasujen ominaisuuksien tutkimiseen. Jatkossa niitä voidaan käyttää myös nesteiden sekä erilaisten bio- ja nanoteknologisten materiaalien tutkimuksessa

Nanoplasmonics simulations at the basis set limit through completeness-optimized, local numerical basis sets

We present an approach for generating local numerical basis sets of improving accuracy for first-principles nanoplasmonics simulations within time-dependent density functional theory. The method is demonstrated for copper, silver, and gold nanoparticles that are of experimental interest but computationally demanding due to the semi-core d-electrons that affect their plasmonic response. The basis sets are constructed by augmenting numerical atomic orbital basis sets by truncated Gaussian-type orbitals generated by the completeness-optimization scheme, which is applied to the photoabsorption spectra of homoatomic metal atom dimers. We obtain basis sets of improving accuracy up to the complete basis set limit and demonstrate that the performance of the basis sets transfers to simulations of larger nanoparticles and nanoalloys as well as to calculations with various exchange-correlation functionals. This work promotes the use of the local basis set approach of controllable accuracy in first-principles nanoplasmonics simulations and beyond.Comment: 11 pages, 6 figure

Tetrahydrofuran Clathrate Hydrate Formation

We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering measurements at the oxygen K edge. A comparison of x-ray Raman spectra measured from water-tetrahydrofuran mixtures and tetrahydrofuran hydrate at different temperatures supports stochastic hydrate formation models rather than models assuming hydrate precursors. This is confirmed by molecular dynamics simulations and density functional theory calculations of x-ray Raman spectra. In addition, changes in the spectra of tetrahydrofuran hydrate with temperatures close to the hydrate's dissociation temperature were observed and may be connected to changes in hydrate's local structure due to the formation of hydrogen bonds between guest and water molecules.We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering measurements at the oxygen K edge. A comparison of x-ray Raman spectra measured from water-tetrahydrofuran mixtures and tetrahydrofuran hydrate at different temperatures supports stochastic hydrate formation models rather than models assuming hydrate precursors. This is confirmed by molecular dynamics simulations and density functional theory calculations of x-ray Raman spectra. In addition, changes in the spectra of tetrahydrofuran hydrate with temperatures close to the hydrate's dissociation temperature were observed and may be connected to changes in hydrate's local structure due to the formation of hydrogen bonds between guest and water molecules.We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering measurements at the oxygen K edge. A comparison of x-ray Raman spectra measured from water-tetrahydrofuran mixtures and tetrahydrofuran hydrate at different temperatures supports stochastic hydrate formation models rather than models assuming hydrate precursors. This is confirmed by molecular dynamics simulations and density functional theory calculations of x-ray Raman spectra. In addition, changes in the spectra of tetrahydrofuran hydrate with temperatures close to the hydrate's dissociation temperature were observed and may be connected to changes in hydrate's local structure due to the formation of hydrogen bonds between guest and water molecules.Peer reviewe