We present an implicit solvation approach where the interface between the quantum-mechanical solute and the surrounding environment is described by a fully continuous permittivity built up with atomic-centered "soft" spheres. This approach combines many of the advantages of the self-consistent continuum solvation model in handling solutes and surfaces in contact with complex dielectric environments or electrolytes in electronic-structure calculations. In addition it is able to describe accurately both neutral and charged systems. The continuous function, describing the variation of the permittivity, allows to compute analytically the nonelectrostatic contributions to the solvation free energy that are described in terms of the quantum surface. The whole methodology is computationally stable, provides consistent energies and forces, and keeps the computational efforts and runtimes comparable to those of standard vacuum calculations. The capabilitiy to treat arbitrary molecular or slab-like geometries as well as charged molecules is key to tackle electrolytes within mixed explicit/implicit frameworks. We show that, with given, fixed atomic radii, two parameters are sufficient to give a mean absolute error of only 1.12 kcal/mol with respect to the experimental aqueous solvation energies for a set of 274 neutral solutes. For charged systems, the same set of parameters provides solvation energies for a set of 60 anions and 52 cations with an error of 2.96 and 2.13 kcal/mol, respectively, improving upon previous literature values. To tackle elements not present in most solvation databases, a new benchmark scheme on wettability and contact angles is proposed for solid-liquid interfaces and applied to the investigation of the stable terminations of a CdS (112̅0) surface in an electrochemical medium.

The computational study of chemical reactions in complex, wet environments is critical for applications in many fields. It is often essential to study chemical reactions in the presence of applied electrochemical potentials, taking into account the non-trivial electrostatic screening coming from the solvent and the electrolytes. As a consequence, the electrostatic potential has to be found by solving the generalized Poisson and the Poisson-Boltzmann equations for neutral and ionic solutions, respectively. In the present work, solvers for both problems have been developed. A preconditioned conjugate gradient method has been implemented for the solution of the generalized Poisson equation and the linear regime of the Poisson-Boltzmann, allowing to solve iteratively the minimization problem with some ten iterations of the ordinary Poisson equation solver. In addition, a self-consistent procedure enables us to solve the non-linear Poisson-Boltzmann problem. Both solvers exhibit very high accuracy and parallel efficiency and allow for the treatment of periodic, free, and slab boundary conditions. The solver has been integrated into the BigDFT and Quantum-ESPRESSO electronic-structure packages and will be released as an independent program, suitable for integration in other codes. C 2016 AIP Publishing LLC. [http://dx

Continuum models to handle solvent and electrolyte effects in an effective way have a long tradition in quantum-chemistry simulations and are nowadays also being introduced in computational condensed-matter and materials simulations. A key ingredient of continuum models is the choice of the solute cavity, i.e. the definition of the sharp or smooth boundary between the regions of space occupied by the quantum-mechanical (QM) system and the continuum embedding environment. The cavity, which should really reflect the region of space accessible to the degrees of freedom of the environmental components (the solvent), is usually defined by an exclusion approach in terms 1 arXiv:1901.08138v1 [physics.chem-ph] 23 Jan 2019 of the degrees of freedom of the system (the solute); typically the atomic position of the QM system or its electronic density. Although most of the solute-based approaches developed lead to models with comparable and high accuracy when applied to small organic molecules, they can introduce significant artifacts when complex systems are considered. As an example, condensed-matter simulations often deal with supports that present open structures, i.e. low-density materials that have regions of space in which a continuum environment could penetrate, while a real solvent would not be able to. Similarly, unphysical pockets of continuum solvent may appear in systems featuring multiple molecular components, e.g. when dealing with hybrid QM/continuum approaches to solvation that involve introducing explicit solvent molecules around the solvated system. Here, we introduce a solvent-aware approach to eliminate the unphysical effects where regions of space smaller than the size of a single solvent molecule could still be filled with a continuum environment. We do this by defining a smoothly varying solute cavity that overcomes several of the limitations of straightforward solute-based definitions. This new approach applies to any smooth local definition of the continuum interface, being it based on the electronic density or the atomic positions of the QM system. It produces boundaries that are continuously differentiable with respect to the QM degrees of freedom, leading to accurate forces and/or Kohn-Sham potentials.The additional parameters involved in the solvent-aware interfaces can be set according to geometrical principles or can be converged to improve accuracy in complex multicomponent systems. Benchmarks on semiconductor substrates and on explicit water substrates confirm the flexibility and the accuracy of the approach and provide a general set of parameters for condensed-matter systems featuring open structures and/or explicit liquid components.

Continuum models have a long tradition in computational chemistry, where they have provided a compact and efficient way to characterize environment effects in quantum-mechanical simulations of solvated systems. Fattebert and Gygi pioneered the development of continuum dielectric embedding schemes for periodic systems and their seamless extension toward molecular dynamics simulations. Following their work, continuum embedding approaches in condensedmatter simulations have thrived. The possibility to model wet and electrified interfaces, with a reduced computational overhead with respect to isolated systems, is opening new perspectives in the characterization of materials and devices. Important applications of these new techniques are in the field of catalysis, electro-chemistry, electro-catalysis, etc. Here we will address the main physical and computational aspects of continuum embedding schemes recently developed for condensed-matter simulations, underlying their peculiarities and their differences with respect to the quantum-chemistry state-of-the-art.condensed matter, continuum models, electro-chemistry, materials, solvation 1 | INTRODUCTION An embedding liquid environment can substantially affect the electronic properties, geometries and spectroscopic responses of the embedded system. Dealing with this kind of effects is a difficult computational task, due to the multiscale nature of the system. A brute force approach to handle these effects would require including all of the solvent atomistic details in the quantum-mechanical (QM) calculation of the solvated system. The total number of degrees of freedom, in particular the number of electrons that need to be described in a first-principles calculation, can thus increase by orders of magnitude. As a result, a calculation that would be feasible in vacuum can easily become untreatable in solution. Moreover, statistical sampling and averaging of the results over different configurations of the liquid would be required, thus further increasing the computational cost of such an approach. These limitations have motivated the development of incredibly powerful simulation programs, [1][2][3] able to take advantage of the rapid evolution of scientific computing hardware and software, for example, by fully exploiting parallel and hybrid architectures. Regardless of its computational feasibility, the explicit description of liquid environments may also suffer from some well-known limitations of current state-of-the-art ab initio methods, in particular regarding their structural [4][5][6][7] and dielectric [8][9][10] properties.On the other hand, when looking at the properties of a solvated system, it is often the case that the detailed effects caused by solute-solvent interactions are less important than the intrinsic properties of the solute. If this is the case, one can exploit a hierarchical approach, where solvent degrees of freedom are treated with a computationally less expensive technique, but are coupled with an highly accurate description of the solvated system...

The BigDFT project started in 2005 with the aim of testing the advantages of using a Daubechies wavelet basis set for Kohn-Sham density functional theory with pseudopotentials. This project led to the creation of the BigDFT code, which employs a computational approach with optimal features for exibility, performance and precision of the results. In particular, the employed formalism has enabled the implementation of an algorithm able to tackle DFT calculations of large systems, up to many thousands of atoms, with a computational eort which scales linearly with the number of atoms. In this work we recall some of the features that have been made possible by the peculiar properties of Daubechies wavelets. In particular, we focus our attention on the usage of DFT for large-scale systems. We show how the localised description of the KS problem, emerging from the features of the basis set, are helpful in providing a simplied description of large-scale electronic structure calculations. We provide some examples on how such simplied description can be employed, and we consider, among the case-studies, the SARS-CoV-2 main protease.

Halide perovskites containing a mixture of formamidinium (FA + ), methylammonium (MA + ) and cesium (Cs + ) cations are the actual standard for obtaining record-efficiency perovskite solar cells. Although the compositional tuning that brings to optimal performance of the devices has been largely established, little is understood on the role of even small quantities of MA + or Cs + in stabilizing the black phase of FAPbI 3 while boosting its photovoltaic yield. In this paper, we use Car−Parrinello molecular dynamics in large supercells containing different ratios of FA + and either MA + or Cs + , in order to study the structural and kinetic features of mixed perovskites at room temperature. Our analysis shows that cation mixing relaxes the rotational disorder of FA + molecules by preferentially aligning their axis toward ⟨100⟩ cubic directions. The phenomenon stems from the introduction of additional local minima in the energetic landscape, which are absent in pure FAPbI 3 crystals. As a result, a higher structural order is achieved, characterized by a pronounced octahedral tilting and a lower vibrational activity for the inorganic framework. We show that both MA + and Cs + are qualified for this enhancement, with Cs + being particularly effective when diluted within the FAPbI 3 perovskite.

Titanium dioxide exhibits superior photocatalytic properties, mainly occurring in liquid environments through molecular adsorptions and dissociations at the solid/liquid interface. The presence of these wet environments is often neglected when performing ab initio calculations for the interaction between the adsorbed molecules and the TiO 2 1 surface. In this study we consider two solvents, i.e. water and ethanol, and show that the proper inclusion of the wet environment in the methodological scheme is fundamental for obtaining reliable results. Our calculations are based on structure predictions at a density functional theory level for molecules interacting with the perfect and defective anatase (1 0 1) surface under both vacuum and wet conditions. A soft-sphere implicit solvation model is used to describe the polar character of the two solvents. As a result, we find that surface oxygen vacancies become energetically favorable with respect to subsurface vacancies at the solid/liquid interface. This aspect is confirmed by ab initio molecular dynamics simulations with explicit water molecules. Ethanol molecules are able to strongly passivate these vacancies, whereas water molecules only weakly interact with the (1 0 1) surface, allowing the coexistence of surface vacancy defects and adsorbed species. Infrared and photoluminescence spectra of anatase nanoparticles exposing predominantly (1 0 1) surfaces dispersed in water and ethanol support the predicted molecular-surface interactions, validating the whole computational paradigm. The combined analysis allows for a better interpretation of TiO 2 processes in wet environments based on improved computational models with implicit solvation features.

Damage evolution and dopant distribution during nanosecond laser thermal annealing of ion implanted silicon have been investigated by means of transmission electron microscopy, secondary ion mass spectrometry, and atom probe tomography. Different melting front positions were realized and studied: nonmelt, partial melt, and full melt with respect to the as-implanted dopant profile. In both boron and silicon implanted silicon samples, the most stable form among the observed defects is that of dislocation loops lying close to (001) and with Burgers vector parallel to the [001] direction, instead of conventional {111} dislocation loops or {311} rod-like defects, which are known to be more energetically favorable and are typically observed in ion implanted silicon. The observed results are explained in terms of a possible modification of the defect formation energy induced by the compressive stress developed in the nonmelted regions during laser annealing.