| 1. |
- Aquilante, Francesco, et al.
(författare)
-
Modern quantum chemistry with [Open]Molcas
- 2020
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:21
-
Tidskriftsartikel (refereegranskat)abstract
- MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.
|
|
| 2. |
- Dahlström, J. Marcus, et al.
(författare)
-
Photoionization Time Delays
- 2016
-
Ingår i: Ultrafast Dynamics Driven by Intense Light Pulses. - Cham : Springer. - 9783319201726 - 9783319201733 ; , s. 177-202
-
Bokkapitel (refereegranskat)abstract
- The material presented in this chapter is based on important advances realized in attophysics which make feasible to follow the motion of electrons in atoms and molecules with attosecond-level time resolution. In this context, time-delays have been recently determined in the process of photoionization by extreme-ultra- violet (XUV) pulses and the question of the significance of these measured delays arises. As we shall outline here, numerical experiments show that they are intimately related to the structure of the ionized species' continuous spectrum. Another point addressed here is that, in experiments, the measurements have the common characteristic to be performed in the presence of an auxiliary infra-red (IR) field, used to clock the timing of the process. This implies to adapt the theory treatment to handle such two-color photoionization processes. We review a systematic analysis of these features that are characteristic of this class of electronic transitions, when viewed in the time domain.
|
|
| 3. |
- Delcey, Mickael G, et al.
(författare)
-
Efficient calculations of a large number of highly excited states for multiconfigurational wavefunctions
- 2019
-
Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 40:19, s. 1789-1799
-
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.
|
|
| 4. |
- Fernández Galván, Ignacio, 1977-, et al.
(författare)
-
Chemiexcitation without the Peroxide Bond? : Replacing oxygen with other heteroatoms
- 2019
-
Ingår i: ChemPhotoChem. - : Wiley. - 2367-0932. ; 3:9, s. 957-967
-
Tidskriftsartikel (refereegranskat)abstract
- Chemiexcitation is the population of electronic excited states from the electronic ground state via radiationless non-adiabatic transitions upon thermal activation. The subsequent emission of the excess of energy in the form of light is called chemiluminescence or bioluminescence when occurring in living organisms. Key intermediates in these reactions have been shown to contain a high-energy (often cyclic) peroxide which decomposes. The simplest molecules, 1,2-dioxetane and 1,2-dioxetanone, have thus been used extensively both theoretically and experimentally as model systems to understand the underlying mechanisms of chemiexcitation. An outstanding question remains whether the peroxide bond is a necessity and whether equivalent processes could happen in other simple molecules not containing an OO bond. In the present work, the decomposition reactions of four analogs of 1,2-dioxetane not containing a peroxide bond, the 1,2-oxazetidine anion, the 1,2-diazetidine anion, (neutral) 1,2-oxazetidine and 1,2-dithietane, have been studied theoretically using ab initio multicongurational methods. In particular, the reaction energy barriers and spin-orbit coupling strengths were calculated; the electronic degeneracy was studied and compared to the case of 1,2-dioxetane to assess the potentiality of chemiexcitation in the analog molecules.
|
|
| 5. |
- Fernández Galván, Ignacio, 1977-, et al.
(författare)
-
Role of conical intersection seam topography in the chemiexcitation of 1,2-dioxetanes
- 2022
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 24:3, s. 1638-1653
-
Tidskriftsartikel (refereegranskat)abstract
- Chemiexcitation, the generation of electronic excited states by a thermal reaction initiated on the ground state, is an essential step in chemiluminescence, and it is mediated by the presence of a conical intersection that allows a nonadiabatic transition from ground state to excited state. Conical intersections classified as sloped favor chemiexcitation over ground state relaxation. The chemiexcitation yield of 1,2-dioxetanes is known to increase upon methylation. In this work we explore to which extent this trend can be attributed to changes in the conical intersection topography or accessibility. Since conical intersections are not isolated points, but continuous seams, we locate regions of the conical intersection seams that are close to the configuration space traversed by the molecules as they react on the ground state. We find that conical intersections are energetically and geometrically accessible from the reaction trajectory, and that topographies favorable to chemiexcitation are found in all three molecules studied. Nevertheless, the results suggest that dynamic effects are more important for explaining the different yields than the static features of the potential energy surfaces.
|
|
| 6. |
- Galván, Ignacio Fdez., et al.
(författare)
-
OpenMolcas : From Source Code to Insight
- 2019
-
Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 15:11, s. 5925-5964
-
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.
|
|
| 7. |
- Hase, Florian, et al.
(författare)
-
Machine learning for analysing ab initio molecular dynamics simulations
- 2020
-
Ingår i: 31St International Conference On Photonic, Electronic And Atomic Collisions (Icpeac Xxxi). - : IOP PUBLISHING LTD.
-
Konferensbidrag (refereegranskat)abstract
- Post-calculation analyses are often required to extract physical insights from ab initio molecular dynamics simulations. In the present work, we use different machine learning classifiers to take a new perspective on the decomposition reaction of dioxetane. Upon thermally activated decomposition, dioxetane can form products in an electronically excited state and can thus chemiluminesce. Simulated dynamics trajectories exhibit both successful and frustrated dissociations. As an exhaustive and systematic study of the decomposition mechanism "by hand" is beyond feasibility, machine learning models have been employed to study the relevant nuclear distortions governing molecular dissociation. According to all classifiers used in the study, the two sets of geometries differ by the in-phase planarisation of the two formaldehyde moieties. New insights are obtained from this analysis: if both moieties are not planar enough when the dissociation is attempted, it is frustrated and the molecule remains trapped. The postponing of the decomposition reaction by the so-called entropic trap enhances the chemiexcitation efficiency.
|
|
| 8. |
- Häse, Florian, et al.
(författare)
-
How machine learning can assist the interpretation of ab initio molecular dynamics simulations and conceptual understanding of chemistry
- 2019
-
Ingår i: Chemical Science. - : The Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 10:8, s. 2298-2307
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations are often key to the understanding of the mechanism, rate and yield of chemical reactions. One current challenge is the in-depth analysis of the large amount of data produced by the simulations, in order to produce valuable insight and general trends. In the present study, we propose to employ recent machine learning analysis tools to extract relevant information from simulation data without a priori knowledge on chemical reactions. This is demonstrated by training machine learning models to predict directly a specific outcome quantity of ab initio molecular dynamics simulations - the timescale of the decomposition of 1,2-dioxetane. The machine learning models accurately reproduce the dissociation time of the compound. Keeping the aim of gaining physical insight, it is demonstrated that, in order to make accurate predictions, the models evidence empirical rules that are, today, part of the common chemical knowledge. This opens the way for conceptual breakthroughs in chemistry where machine analysis would provide a source of inspiration to humans.
|
|
| 9. |
- Jenkins, Andrew J., et al.
(författare)
-
The Ehrenfest method with fully quantum nuclear motion (Qu-Eh) : Application to charge migration in radical cations
- 2018
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 149:9
-
Tidskriftsartikel (refereegranskat)abstract
- An algorithm is described for quantum dynamics where an Ehrenfest potential is combined with fully quantum nuclear motion (Quantum-Ehrenfest, Qu-Eh). The method is related to the single-set variational multi-configuration Gaussian approach (vMCG) but has the advantage that only a single quantum chemistry computation is required at each time step since there is only a single time-dependent potential surface. Also shown is the close relationship to the “exact factorization method.” The quantum Ehrenfest method is compared with vMCG for study of electron dynamics in a modified bismethylene-adamantane cation system. Illustrative examples of electron-nuclear dynamics are presented for a distorted allene system and for HCCI+ where one has a degenerate Π system.
|
|
| 10. |
|
|
