Electronic Structure of Water from Koopmans-Compliant Functionals

Predicting electronic screening for fast Koopmans spectral functional calculations

Koopmans spectral functionals are a powerful extension of Kohn-Sham density-functional theory (DFT) that enables the prediction of spectral properties with state-of-the-art accuracy. The success of these functionals relies on capturing the effects of electronic screening through scalar, orbital-dependent parameters. These parameters have to be computed for every calculation, making Koopmans spectral functionals more expensive than their DFT counterparts. In this work, we present a machine-learning model that-with minimal training-can predict these screening parameters directly from orbital densities calculated at the DFT level. We show in two prototypical use cases that using the screening parameters predicted by this model, instead of those calculated from linear response, leads to orbital energies that differ by less than 20 meV on average. Since this approach dramatically reduces run times with minimal loss of accuracy, it will enable the application of Koopmans spectral functionals to classes of problems that previously would have been prohibitively expensive, such as the prediction of temperature-dependent spectral properties. More broadly, this work demonstrates that measuring violations of piecewise linearity (i.e., curvature in total energies with respect to occupancies) can be done efficiently by combining frozen-orbital approximations and machine learning.

Synergistic Interaction of Hyperbranched Polyglycerols and Cetyltrimethylammonium Bromide for Oil/Water Interfacial Tension Reduction: A Molecular Dynamics Study

Applying surfactants to reduce the interfacial tension (IFT) on water/oil interfaces is a proven technique. The search for new surfactants and delivery strategies is an ongoing research area with applications in many fields such as drug delivery through nanoemulsions and enhanced oil recovery. Experimentally, the combination of hyperbranched polyglycerol (HPG) with cetyltrimethylammonium bromide (CTAB) substantially reduced the observed IFT of oil/water interface, 0.9 mN/m, while HPG alone was 5.80 mN/m and CTAB alone IFT was 8.08 mN/m. Previous simulations in an aqueous solution showed that HPG is a surfactant carrier. Complementarily, in this work, we performed classical molecular dynamics simulations on combinations of CTAB and HPG with one aliphatic chain to investigate further the interaction of this pair in oil interfaces and propose the mechanism of IFT decrease. Basically, from our results, one can observe that the IFT reduction comes from a combination of effects that have not been observed for other dual systems: (i) Due to the CTAB-HPG strong interaction, a weakening of their specific and isolated interactions with the water and oil phases occurs. (ii) Aggregates enlarge the interfacial area, turning it into a less ordered interface. (iii) The spread of individual molecules charge profiles leads to the much lower interfacial tension observed with the CTAB+HPG systems.

koopmans: An Open-Source Package for Accurately and Efficiently Predicting Spectral Properties with Koopmans Functionals

Over the past decade we have developed Koopmans functionals, a computationally efficient approach for predicting spectral properties with an orbital-density-dependent functional framework. These functionals impose a generalized piecewise linearity condition to the entire electronic manifold, ensuring that orbital energies match the corresponding electron removal/addition energy differences (in contrast to semilocal DFT, where a mismatch between the two lies at the heart of the band gap problem and, more generally, the unreliability of Kohn-Sham orbital energies). This strategy has proven to be very powerful, yielding molecular orbital energies and solid-state band structures with comparable accuracy to many-body perturbation theory but at greatly reduced computational cost while preserving a functional formulation. This paper reviews the theory of Koopmans functionals, discusses the algorithms necessary for their implementation, and introduces koopmans, an open-source package that contains all of the code and workflows needed to perform Koopmans functional calculations and obtain reliable spectral properties of molecules and materials.

Theoretical investigation of Aryl/Alkyl halide reduction with hydrated electrons from energy and AIMD aspects

CONTEXT

In this study, the reactions of hydrated electron (e^-(aq)) with alkyl and aryl halides were simulated with an ab initial molecular dynamics (AIMD) method to reveal the underlying mechanism. An original protocol was developed for preparing the proper initial wavefunction guess of AIMD, in which a single electron was curled in a tetrahedral cavity of four water molecules. Our results show that the stability of e^-(aq) increases with the hydrogen bond grid integrity. The organic halides prefer to react with e^-(aq) in neutral or alkaline environment, while they are more likely to react with hydrogen radical (the product of e^-(aq) and proton) under acidic conditions. The reaction between fluorobenzene/fluoromethane and hydrogen radical is considered as the least favorable reaction due to the highest reaction barriers. The bond dissociation energy (BDE) suggested that the cleavage of the carbon-halogen bond of their anion radical might be a thermodynamically favorable reaction. AIMD results indicated that the LUMO or higher orbitals were the e^-(aq) migration destination. The transplanted electron enhanced carbon-halogen bond vibration intensively, leading to bond cleavage. The solvation process of the departing halogen anions was observed in both fluorobenzene and fluoromethane AIMD simulation, indicating that it might have a significant effect on enthalpy. Side reactions and byproducts obtained during the AIMD simulation suggested the complexity of the e^-(aq) reactions and further investigation was needed to fully understand the reaction mechanisms. This study provided theoretical insight into the pollutant environmental fate and constructed a methodological foundation for AIMD simulation of analogous free radical reactions.

METHODS

The theoretical calculation was conducted on the combination of Gaussian16 and ORCA5.0.3 software packages. The initial geometries, as well as the wavefunction initial guesses, were obtained at PBE0/ma-def2-TZVP/IEFPCM-water level in Gaussian16 unless otherwise stated. AIMD simulations were performed at the same level in ORCA. Wavefunction analysis was carried out with Multiwfn. The details methods were described in the section "Computational details" section.

Koopmans Spectral Functionals in Periodic Boundary Conditions

Koopmans spectral functionals aim to describe simultaneously ground-state properties and charged excitations of atoms, molecules, nanostructures, and periodic crystals. This is achieved by augmenting standard density functionals with simple but physically motivated orbital-density-dependent corrections. These corrections act on a set of localized orbitals that, in periodic systems, resemble maximally localized Wannier functions. At variance with the original, direct supercell implementation (Phys. Rev. X 2018, 8, 021051), we discuss here (i) the complex but efficient formalism required for a periodic boundary code using explicit Brillouin zone sampling and (ii) the calculation of the screened Koopmans corrections with density functional perturbation theory. In addition to delivering improved scaling with system size, the present development makes the calculation of band structures with Koopmans functionals straightforward. The implementation in the open-source Quantum ESPRESSO distribution and the application to prototypical insulating and semiconducting systems are presented and discussed.