A general framework for active space embedding methods with applications in quantum computing.

We developed a general framework for hybrid quantum-classical computing of molecular and periodic embedding approaches based on an orbital space separation of the fragment and environment degrees of freedom. We demonstrate its potential by presenting a specific implementation of periodic range-separated DFT coupled to a quantum circuit ansatz, whereby the variational quantum eigensolver and the quantum equation-of-motion algorithm are used to obtain the low-lying spectrum of the embedded fragment Hamiltonian. The application of this scheme to study localized electronic states in materials is showcased through the accurate prediction of the optical properties of the neutral oxygen vacancy in magnesium oxide (MgO).

Efficient random phase approximation for diradicals.

We apply the analytically solvable model of two electrons in two orbitals to diradical molecules, characterized by two unpaired electrons. The effect of doubly occupied and empty orbitals is taken into account by means of random phase approximation (RPA). We show that in the static limit, the direct RPA leads to the renormalization of the parameters of the two-orbital model.

Multi-Level Protocol for Mechanistic Reaction Studies Using Semi-Local Fitted Potential Energy Surfaces.

In this work, we propose a multi-level protocol for routine theoretical studies of chemical reaction mechanisms. The initial reaction paths of our investigated systems are sampled using the Nudged Elastic Band (NEB) method driven by a cheap electronic structure method. Forces recalculated at the more accurate electronic structure theory for a set of points on the path are fitted with a machine learning technique (in our case symmetric gradient domain machine learning or sGDML) to produce a semi-local reactive potential energy surface (PES), embracing reactants, products and transition state (TS) regions.

Temperature Dependent Properties of the Aqueous Electron.

The temperature-dependent properties of the aqueous electron have been extensively studied using mixed quantum-classical simulations in a wide range of thermodynamic conditions based on one-electron pseudopotentials. While the cavity model appears to explain most of the physical properties of the aqueous electron, only a non-cavity model has so far been successful in accounting for the temperature dependence of the absorption spectrum. Here, we present an accurate and efficient description of the aqueous electron under various thermodynamic conditions by combining hybrid functional-based molecular dynamics, machine learning techniques, and multiple time-step methods.

Shallow and deep trap states of solvated electrons in methanol and their formation, electronic excitation, and relaxation dynamics.

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (e ) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions.

Surface tension of liquids and binary mixtures from molecular dynamics simulations.

In this work we assess and extend strategies for calculating surface tension of complex liquids from molecular dynamics simulations: the mechanical route and the instantaneous liquid interface (ILI) approach. The former employs the connection between stress tensor and surface tension, whereas the latter involves computation of instantaneous density field. Whereas the mechanical route is general, the ILI method involves system-dependent parameters restricting its original application to liquid water only.

Subsystem Density Functional Theory Augmented by a Delta Learning Approach to Achieve Kohn-Sham Accuracy.

Simulations based on electronic structure theory naturally include polarization and have no transferability problems. In particular, Kohn-Sham density functional theory (KS-DFT) has become the method of reference for ab initio molecular dynamics simulations of condensed matter systems. However, the high computational cost often poses strict limits on the affordable system size as well as on the extension of sampling (number of configurations).

Formulation and Implementation of Density Functional Embedding Theory Using Products of Basis Functions.

The representation of embedding potential using products of atomic orbital basis functions has been developed in the context of density functional embedding theory. The formalism allows to treat pseudopotential and all-electron calculations on the same footing and enables simple transfer of the embedding potential in a compact matrix form. In addition, a cost-reduction procedure for the basis set and potential reduction based on population analysis has been proposed.

CP2K: An electronic structure and molecular dynamics software package - Quickstep: Efficient and accurate electronic structure calculations.

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems.

Simulating the ghost: quantum dynamics of the solvated electron.

The nature of the bulk hydrated electron has been a challenge for both experiment and theory due to its short lifetime and high reactivity, and the need for a high-level of electronic structure theory to achieve predictive accuracy. The lack of a classical atomistic structural formula makes it exceedingly difficult to model the solvated electron using conventional empirical force fields, which describe the system in terms of interactions between point particles associated with atomic nuclei. Here we overcome this problem using a machine-learning model, that is sufficiently flexible to describe the effect of the excess electron on the structure of the surrounding water, without including the electron in the model explicitly.

Decomposition of сarbon tetrachloride under the action of a dielectric barrier discharge of atmospheric pressure in an oxygen atmosphere.

In this study, the process of decomposition of carbon tetrachloride (CCl) vapor in oxygen DBD at atmospheric pressure and its kinetic regularities have been studied. In the course of the experiments, it was shown that the efficiency of the decomposition of carbon tetrachloride in DBD can reach 100%. Depending on the conditions of the experiments, the effective rate constants were equal to (0.

Double-Hybrid DFT Functionals for the Condensed Phase: Gaussian and Plane Waves Implementation and Evaluation.

Intermolecular interactions play an important role for the understanding of catalysis, biochemistry and pharmacy. Double-hybrid density functionals (DHDFs) combine the proper treatment of short-range interactions of common density functionals with the correct description of long-range interactions of wave-function correlation methods. Up to now, there are only a few benchmark studies available examining the performance of DHDFs in condensed phase.

Mechanism of Aqueous Carbon Dioxide Reduction by the Solvated Electron.

Aqueous solvated electron (e), a key species in radiation and plasma chemistry, can efficiently reduce CO in a potential green chemistry application. Here, the mechanism of this reaction is unravelled by condensed-phase molecular dynamics based on the correlated wave function and an accurate density functional theory (DFT) approximation. Here, we design and apply the holistic protocol for solvated electron's reactions encompassing all relevant reaction stages starting from diffusion.

Ionization of Water as an Effect of Quantum Delocalization at Aqueous Electrode Interfaces.

The enhanced probability of water dissociation at the aqueous electrode interfaces is predicted by path-integral molecular dynamics. The ionization process is observed at the aqueous platinum interface when nuclear quantum effects are introduced in the statistical sampling, while minor effects have been observed at the gold interface. We characterize the dissociation mechanism of the formed water ions.

Distribution of Policyclic aromatic hydrocarbons in a snow cover in the territory of Ivanovo city, Russia.

The paper presents the results of a study of the content of 12 polyaromatic hydrocarbons (PAHs) in the snow cover of the city of Ivanovo (Russian Federation). It is shown that their average content exceeds the background level by 6.6 times, which made it possible to identify for which compounds the admission channels are associated with transboundary transport (naphthalene, pyrene, benz [b]fluorantin, benzo [a]pyrene and dibenz [a,h]anthracene), and for which with local emission sources (anthracene, phenanthrene, fluoranthene, chrysene, benz [k]fluorantin, and benzo [g,h,i]perylene).

Sampling Potential Energy Surfaces in the Condensed Phase with Many-Body Electronic Structure Methods.

Sampling potential energy surfaces (PES) is pivotal for understanding chemical structure, energetics and reactivity and is of special importance for complex condensed-phase systems. Until recently such simulations based on electronic structure theory have been performed only by density functional theory and semiempirical methods. Many-body electronic structure methods, almost routinely used for molecules, have been practically unavailable for sampling PES in the condensed-phase.

Dynamics of the Bulk Hydrated Electron from Many-Body Wave-Function Theory.

The structure of the hydrated electron is a matter of debate as it evades direct experimental observation owing to the short life time and low concentrations of the species. Herein, the first molecular dynamics simulation of the bulk hydrated electron based on correlated wave-function theory provides conclusive evidence in favor of a persistent tetrahedral cavity made up by four water molecules, and against the existence of stable non-cavity structures. Such a cavity is formed within less than a picosecond after the addition of an excess electron to neat liquid water, with less regular cavities appearing as intermediates.

Franck-Condon Theory of Quantum Mechanochemistry.

The spectroscopic Franck-Condon (FC) principle is extended to mechanochemistry. If the external force is applied rapidly (the sudden-force regime), then the transition between the potential energy surface and the force-modified potential energy surface is analogous to the optical electronic transition. Such a transition produces a nonequilibrium ensemble of vibrationally excited molecules.

Gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene: cog-wheel-type vs. independent internal rotation and influence of dispersion interactions.

The gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene (1,8-BTMSA) was determined by a combined gas electron diffraction (GED)/mass spectrometry (MS) experiment as well as by quantum-chemical calculations (QC). DFT and dispersion corrected DFT calculations (DFT-D3) predicted two slightly different structures for 1,8-BTMSA concerning the mutual orientation of the two -C-C[triple bond, length as m-dash]C-SiMe units: away from one another or both bent to the same side. An attempt was made to distinguish these structures by GED structural analysis.

Nuclear Quantum Effects on Aqueous Electron Attachment and Redox Properties.

Nuclear quantum effects (NQEs) on the reduction and oxidation properties of small aqueous species (CO, HO, and O) are quantified and rationalized by first-principles molecular dynamics and thermodynamic integration. Vertical electron attachment, or electron affinity, and detachment energies (VEA and VDE) are strongly affected by NQEs, decreasing in absolute value by 0.3 eV going from a classical to a quantum description of the nuclei.

Application of classical simulations for the computation of vibrational properties of free molecules.

In this study, we investigate the ability of classical molecular dynamics (MD) and Monte-Carlo (MC) simulations for modeling the intramolecular vibrational motion. These simulations were used to compute thermally-averaged geometrical structures and infrared vibrational intensities for a benchmark set previously studied by gas electron diffraction (GED): CS, benzene, chloromethylthiocyanate, pyrazinamide and 9,12-I-1,2-closo-CBH. The MD sampling of NVT ensembles was performed using chains of Nose-Hoover thermostats (NH) as well as the generalized Langevin equation thermostat (GLE).

The effect of molecular dynamics sampling on the calculated observable gas-phase structures.

In this study, we compare the performance of various ab initio molecular dynamics (MD) sampling methods for the calculation of the observable vibrationally-averaged gas-phase structures of benzene, naphthalene and anthracene molecules. Nose-Hoover (NH), canonical and quantum generalized-Langevin-equation (GLE) thermostats as well as the a posteriori quantum correction to the classical trajectories have been tested and compared to the accurate path-integral molecular dynamics (PIMD), static anharmonic vibrational calculations as well as to the experimental gas electron diffraction data. Classical sampling methods neglecting quantum effects (NH and canonical GLE thermostats) dramatically underestimate vibrational amplitudes for the bonded atom pairs, both C-H and C-C, the resulting radial distribution functions exhibit nonphysically narrow peaks.

Spin-Unrestricted Second-Order Møller-Plesset (MP2) Forces for the Condensed Phase: From Molecular Radicals to F-Centers in Solids.

To obtain consistent geometries for the computation of properties, nuclear gradients are essential. Here, we report a fully periodic Γ-point, massively parallel implementation of spin-unrestricted second-order Møller-Plesset (MP2) forces. It is based on the resolution-of-identity and Gaussian and plane waves approach to calculate electron repulsion integrals and is made available in the CP2K program.