| 1. |
- Aurbakken, Einar, et al.
(författare)
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Transient spectroscopy from time-dependent electronic-structure theory without multipole expansions
- 2024
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : American Physical Society. - 2469-9926 .- 2469-9934. ; 109:1
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Tidskriftsartikel (refereegranskat)abstract
- Based on the work done by an electromagnetic field on an atomic or molecular electronic system, a general gauge-invariant formulation of transient absorption spectroscopy is presented within the semiclassical approximation. Avoiding multipole expansions, a computationally viable expression for the spectral response function is derived from the minimal-coupling Hamiltonian of an electronic system interacting with one or more laser pulses described by a source-free, enveloped electromagnetic vector potential. With a fixed-basis expansion of the electronic wave function, the computational cost of simulations of laser-driven electron dynamics beyond the dipole approximation is the same as simulations adopting the dipole approximation. We illustrate the theory by time-dependent configuration interaction and coupled-cluster simulations of core-level absorption and circular dichroism spectra.
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| 2. |
- Delcey, Mickael G, et al.
(författare)
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Efficient calculations of a large number of highly excited states for multiconfigurational wavefunctions
- 2019
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Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 40:19, s. 1789-1799
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Tidskriftsartikel (refereegranskat)abstract
- Electronically excited states play important roles in many chemical reactions and spectroscopic techniques. In quantum chemistry, a common technique to solve excited states is the multiroot Davidson algorithm, but it is not designed for processes like X-ray spectroscopy that involves hundreds of highly excited states. We show how the use of a restricted active space wavefunction together with a projection operator to remove low-lying electronic states offers an efficient way to reach single and double-core-hole states. Additionally, several improvements to the stability and efficiency of the configuration interaction (CI) algorithm for a large number of states are suggested. When applied to a series of transition metal complexes the new CI algorithm does not only resolve divergence issues but also leads to typical reduction in computational time by 70%, with the largest savings for small molecules and large active spaces. Together, the projection operator and the improved CI algorithm now make it possible to simulate a wide range of single- and two-photon spectroscopies.
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| 3. |
- Delcey, Mickael G., et al.
(författare)
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Exact semi-classical light-matter interaction operator applied to two-photon processes with strong relativistic effects
- 2020
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 153:2
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Tidskriftsartikel (refereegranskat)abstract
- X-ray processes involve interactions with high-energy photons. For these short wavelengths, the perturbing field cannot be treated as constant, and there is a need to go beyond the electric-dipole approximation. The exact semi-classical light-matter interaction operator offers several advantages compared to the multipole expansion such as improved stability and ease of implementation. Here, the exact operator is used to model x-ray scattering in metal K pre-edges. This is a relativistic two-photon process where absorption is dominated by electric-dipole forbidden transitions. With the restricted active space state-interaction approach, spectra can be calculated even for the multiconfigurational wavefunctions including second-order perturbation. However, as the operator itself depends on the transition energy, the cost for evaluating integrals for hundreds of thousands unique transitions becomes a bottleneck. Here, this is solved by calculating the integrals in a molecular-orbital basis that only runs over the active space, combined with a grouping scheme where the operator is the same for close-lying transitions. This speeds up the calculations of single-photon processes and is critical for the modeling of two-photon scattering processes. The new scheme is used to model Kα resonant inelastic x-ray scattering of iron-porphyrin complexes with relevance to studies of heme enzymes, for which the total computational time is reduced by several orders of magnitude with an effect on transition intensities of 0.1% or less.
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| 4. |
- Ertan, Emelie, et al.
(författare)
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Setting the stage for theoretical X-ray spectra of the H2S molecule with RASPT2 calculations of the energy landscape
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Electronic valence- and core-excitations into the anti-bonding orbitals of the H2S molecule have been calculated within a multi-configurational wave-function framework (RASPT2). Potential energy surfaces and transition dipole moments have been derived in two dimensions for the S-H stretching coordinates. The |S1s-1, 6a11> and |S1s-1, 3b21> core-excited states in H2S are nearly degenerate along the symmetric stretching coordinate, for which we have identified two conical intersections. The small energy splitting of the S1s-1 core-excited states at equilibrium geometry arise from an avoided crossing at broken symmetry. Compared to the water molecule, which exhibit state-selective gating to different vibrational modes [Nat. Commun. 8 14165 (2017)] in its well-separated O1s-1 core-excited states, we expect a strong coupling between the close-lying |S1s-1, 6a11> and |S1s-1, 3b21> states. This could lead to dissociative dynamics observable in K-edge RIXS. The S2p-1 core-excited states form two dense manifolds of spin-orbit coupled states, which can be schematically characterised as bound |S2p-1,3b21> and dissociative |S2p-1,6a11> states. We identify three conical intersections in the singlet and triplet states along the symmetric stretching coordinate. Mapping the molecular singlet and triplet states to the atomic dissociation limit reveals a symmetry selection rule, leading to an off-set of the 1|3a1-1,6a11> state relative to the other S2p-1 core-excited states. The dense manifolds of S2p-1 core-excited states will complicate the analysis of Kα-edge RIXS, but dynamical effects could be evaluated through detuning and in comparison to L-edge XAS. In L-edge RIXS, the dynamical effects well be more pronounced due to a longer life-time of the S2p-1 core-excited states compared to the S1s-1 core-excited states.
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| 5. |
- Galván, Ignacio Fdez., et al.
(författare)
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OpenMolcas : From Source Code to Insight
- 2019
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 15:11, s. 5925-5964
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Tidskriftsartikel (refereegranskat)abstract
- In this Article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics, and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the Article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism, and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with postcalculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory, and new electronic and muonic basis sets.
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| 6. |
- Guo, Meiyuan, et al.
(författare)
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Fingerprinting Electronic Structure of Heme Iron by Ab Initio Modeling of Metal L-Edge X-ray Absorption Spectra
- 2019
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 15:1, s. 477-489
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Tidskriftsartikel (refereegranskat)abstract
- The capability of the multiconfigurational restricted active space approach to identify electronic structure from spectral fingerprints is explored by applying it to iron L-edge X-ray absorption spectroscopy (XAS) of three heme systems that represent the limiting descriptions of iron in the Fe-O-2 bond, ferrous and ferric [Fe(P)(ImH)(2)](0/1+) (P = porphine, ImH = imidazole), and Fe-II(P). The level of agreement between experimental and simulated spectral shapes is calculated using the cosine similarity, which gives a quantitative and unbiased assignment. Further dimensions in fingerprinting are obtained from the L-edge branching ratio, the integrated absorption intensity, and the edge position. The results show how accurate ab initio simulations of metal L-edge XAS can complement calculations of relative energies to identify unknown species in chemical reactions.
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| 7. |
- Guo, Meiyuan, et al.
(författare)
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Molecular orbital simulations of metal 1s2p resonant inelastic X-ray scattering
- 2016
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 120:29, s. 5848-5855
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Tidskriftsartikel (refereegranskat)abstract
- For first-row transition metals, high-resolution 3d electronic structure information can be obtained using resonant inelastic X-ray scattering (RIXS). In the hard X-ray region, a K pre-edge (1s -> 3d) excitation can be followed by monitoring the dipole-allowed K alpha (2p -> 1s) or K beta (3p -> 1s) emission, processes labeled 1s2p or 1s3p RIXS. Here the restricted active space (RAS) approach, which is a molecular orbital method, is used for the first time to study hard X-ray RIXS processes. This is achieved by including the two sets of core orbitals in different partitions of the active space. Transition intensities are calculated using both first- and second-order expansions of the wave vector, including, but not limited to, electric dipoles and quadrupoles. The accuracy of the approach is tested for 1s2p RIXS of iron hexacyanides [Fe(CN)(6)](n-) in ferrous and ferric oxidation states. RAS simulations accurately describe the multiplet structures and the role of 2p and 3d spin-orbit coupling on energies and selection rules. Compared to experiment, relative energies of the two [Fe(CN)(6)](3-) resonances deviate by 0.2 eV in both incident energy and energy transfer directions, and multiplet splittings in [Fe(CN)(6)](4-) are reproduced within 0.1 eV. These values are similar to what can be expected for valence excitations. The development opens the modeling of hard X-ray scattering processes for both solution catalysts and enzymatic systems.
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| 8. |
- Guo, Meiyuan, et al.
(författare)
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Simulations of iron K pre-edge X-ray absorption spectra using the restricted active space method
- 2016
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 4, s. 3250-3259
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Tidskriftsartikel (refereegranskat)abstract
- The intensities and relative energies of metal K pre-edge features are sensitive to both geometric and electronic structures. With the possibility to collect high-resolution spectral data it is important to find theoretical methods that include all important spectral effects: ligand-field splitting, multiplet structures, 3d-4p orbital hybridization, and charge-transfer excitations. Here the restricted active space (RAS) method is used for the first time to calculate metal K pre-edge spectra of open-shell systems, and its performance is tested against on six iron complexes: [FeCl6](n-), [FeCl4](n-), and [Fe(CN)(6)](n-) in ferrous and ferric oxidation states. The method gives good descriptions of the spectral shapes for all six systems. The mean absolute deviation for the relative energies of different peaks is only 0.1 eV. For the two systems that lack centrosymmetry [FeCl4](2-/1-), the ratios between dipole and quadrupole intensity contributions are reproduced with an error of 10%, which leads to good descriptions of the integrated pre-edge intensities. To gain further chemical insight, the origins of the pre-edge features have been analyzed with a chemically intuitive molecular orbital picture that serves as a bridge between the spectra and the electronic structures. The pre-edges contain information about both ligand-field strengths and orbital covalencies, which can be understood by analyzing the RAS wavefunction. The RAS method can thus be used to predict and rationalize the effects of changes in both the oxidation state and ligand environment in a number of hard X-ray studies of small and medium-sized molecular systems.
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| 9. |
- Kragh Sørensen, Lasse, et al.
(författare)
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Nature of the Anomalous Size Dependence of Resonance Red Shifts in Ultrafine Plasmonic Nanoparticles
- 2022
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:39, s. 16804-16814
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Tidskriftsartikel (refereegranskat)abstract
- Plasmonic red shifts of nanoparticles are commonly used in imaging technologies to probe the character of local environments, and the understanding of their dependence on size, shape, and surrounding media has therefore become an important target for research. The red shift of plasmon resonances changes character at about 8-10 nm of size for spherical gold nanoparticles-above this value, the red shift progresses linearly with particle size, while below this size, the red shift changes nonlinearly and more strongly with size. Using an atomistic discrete interaction model, we have studied the special properties of the nanoparticle surface layers and discovered its importance for ultrafine plasmonic nanoparticles and their red shifts. We find that the physical origin for the specific properties inherent to the surface layer of atoms near the nanoparticle boundary is related to the anisotropy of the local environment of atoms in this layer by other atoms. The anisotropy changes the conditions for light-induced nonlocal interactions of neighboring atoms with each other and with the incident radiation compared to the atoms located in the particle core with isotropic nearest surroundings by other atoms. The local anisotropy of the nanoparticle crystal lattice is a geometric factor that increases toward its boundary and that is the most fundamental factor underlying the physical differences between the nanoparticle surface layer and the core material. It is shown that the inflexion point at 8-10 nm is due to a change in the dominant physical origin of the red shift -from chaotization of atomically light-induced dipoles within the surface layer in the case of ultrafine nanoparticles to retardation effects for large nanoparticles in which the relative volume of the surface layer decreases rapidly to a negligible value with increasing nanoparticle size. The patterns revealed are the basis for predicting the manifestation of surface layer effects in ultrafine plasmonic nanoparticles of different and of different materials.
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| 10. |
- Larsson, H. R., et al.
(författare)
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Correlation effects in strong-field ionization of heteronuclear diatomic molecules
- 2016
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Ingår i: PHYSICAL REVIEW A. - 2469-9926. ; 93:1
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Tidskriftsartikel (refereegranskat)abstract
- We develop a time-dependent theory to investigate electron dynamics and photoionization processes of diatomic molecules interacting with strong laser fields including electron-electron correlation effects. We combine the recently formulated time-dependent generalized-active-space configuration interaction theory [D. Hochstuhl and M. Bonitz, Phys. Rev. A 86, 053424 (2012); S. Bauch et al., ibid. 90, 062508 (2014)] with a prolate spheroidal basis set including localized orbitals and continuum states to describe the bound electrons and the outgoing photoelectron. As an example, we study the strong-field ionization of the two-center four-electron lithium hydride molecule in different intensity regimes. By using single-cycle pulses, two orientations of the asymmetric heteronuclear molecule are investigated: Li-H, with the electrical field pointing from H to Li, and the opposite case of H-Li. The preferred orientation for ionization is determined and we find a transition from H-Li, for low intensity, to Li-H, for high intensity. The influence of electron correlations is studied at different levels of approximation, and we find a significant change in the preferred orientation. For certain intensity regimes, even an interchange of the preferred configuration is observed, relative to the uncorrelated simulations. Further insight is provided by detailed comparisons of photoelectron angular distributions with and without correlation effects taken into account.
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