Effective local potentials for density and density-matrix functional approximations with non-negative screening density.

A way to improve the accuracy of the spectral properties in density functional theory (DFT) is to impose constraints on the effective, Kohn-Sham (KS), local potential [J. Chem. Phys.

Evaluating the performance of ReaxFF potentials for sp carbon systems (graphene, carbon nanotubes, fullerenes) and a new ReaxFF potential.

We study the performance of eleven reactive force fields (ReaxFF), which can be used to study sp carbon systems. Among them a new hybrid ReaxFF is proposed combining two others and introducing two different types of C atoms. The advantages of that potential are discussed.

Density inversion method for local basis sets without potential auxiliary functions: inverting densities from RDMFT.

A density inversion method is presented, to obtain the constrained, optimal, local potential that has a prescribed asymptotic behaviour and reproduces optimally any given ground-state electronic density. This work builds upon the method of [Callow , , 2020, , 164114.] and differs in the expansion of the screening density in orbital basis element products instead of basis functions of an additional auxiliary set.

Uniaxially Strained Graphene: Structural Characteristics and G-Mode Splitting.

The potential use of graphene in various strain engineering applications requires an accurate characterization of its properties when the material is under different mechanical loads. In this work, we present the strain dependence of the geometrical characteristics at the atomic level and the Raman active G-band evolution in a uniaxially strained graphene monolayer, using density functional theory methods as well as molecular dynamics atomistic simulations for strains that extend up to the structural failure. The bond length and bond angle variations with strain, applied either along the zigzag or along the armchair direction, are discussed and analytical relations describing this dependence are provided.

Generalized Kohn-Sham equations with accurate total energy and single-particle eigenvalue spectrum.

We propose a new generalized Kohn-Sham or constrained hybrid method, where the exchange potential is the (equally weighted) average of the nonlocal Fock-exchange term and the self-interaction-corrected exchange potential, as obtained from our constrained minimization method of semi-local approximations. The new method gives an accurate single-particle eigenvalue spectrum with an average deviation between (the negative of) the valence orbital eigenvalues and the experimental ionization potentials of about 0.5 eV, while the deviation of core orbitals is within 2 eV.

Width Dependent Elastic Properties of Graphene Nanoribbons.

The mechanical response of graphene nanoribbons under uniaxial tension, as well as its dependence on the nanoribbon width, is presented by means of numerical simulations. Both armchair and zigzag edged graphene nanoribbons are considered. We discuss results obtained through two different theoretical approaches, viz.

Preparation of hydrogen, fluorine and chlorine doped and co-doped titanium dioxide photocatalysts: a theoretical and experimental approach.

Titanium dioxide (TiO) has a strong photocatalytic activity in the ultra-violet part of the spectrum combined with excellent chemical stability and abundance. However, its photocatalytic efficiency is prohibited by limited absorption within the visible range derived from its wide band gap value and the presence of charge trapping states located at the band edges, which act as electron-hole recombination centers. Herein, we modify the band gap and improve the optical properties of TiO via co-doping with hydrogen and halogen.

Improving the exchange and correlation potential in density-functional approximations through constraints.

We review and expand on our work to impose constraints on the effective Kohn-Sham (KS) potential of local and semi-local density-functional approximations. Constraining the minimisation of the approximate total energy density-functional invariably leads to an optimised effective potential (OEP) equation, the solution of which yields the KS potential. We review briefly our previous work on this and demonstrate with numerous examples that despite the well-known mathematical issues of the OEP with finite basis sets, our OEP equations are numerically robust.

Density-inversion method for the Kohn-Sham potential: Role of the screening density.

We present a method to invert a given density and find the Kohn-Sham (KS) potential in Density Functional Theory (DFT) that shares the density. Our method employs the concept of screening density, which is naturally constrained by the inversion procedure and thus ensures that the density being inverted leads to a smooth KS potential with correct asymptotic behavior. We demonstrate the applicability of our method by inverting both local and non-local (Hartree-Fock and coupled cluster) densities; we also show how the method can be used to mitigate the effects of self-interactions in common DFT potentials with appropriate constraints on the screening density.

Defect processes in F and Cl doped anatase TiO.

Titanium dioxide represents one of the most widely studied transition metal oxides due to its high chemical stability, non-toxicity, abundance, electron transport capability in many classes of optoelectronic devices and excellent photocatalytic properties. Nevertheless, the wide bang gap of pristine oxide reduces its electron transport ability and photocatalytic activity. Doping with halides and other elements has been proven an efficient defect engineering strategy in order to reduce the band gap and maximize the photocatalytic activity.

Interfacing tetrapyridyl-C with porphyrin dimers via π-conjugated bridges: artificial photosynthetic systems with ultrafast charge separation.

We report on the synthesis, characterization and photophysical properties of a donor-bridge-acceptor supramolecular hybrid system, consisting of a tetrapyridyl fullerene derivative (C60-tpyr) as electron acceptor, with the four pyridyl groups as part of oligophenyleneethynylene/phenylenevinylene bridges, and zinc porphyrin dimers (ZnP)2 as electron donor species. Based on the metal-to-ligand coordination between the zinc metal centers of (ZnP)2 and the four pyridyl entities of C60-tpyr, a strong binding constant (5 × 105 M-1) for the formation of C60-tpyr·[(ZnP)2]2 was evidenced. Insights into the electronic interactions between the photoactive (ZnP)2 units and C60-tpyr emanated from complementary physicochemical assays, which were further supported by theoretical calculations.

Structure of the first order reduced density matrix in three electron systems: A generalized Pauli constraints assisted study.

We investigate the structure of the one-body reduced density matrix of three electron systems, i.e., doublet and quadruplet spin configurations, corresponding to the smallest interacting system with an open-shell ground state.

Atomistic potential for graphene and other sp carbon systems.

We introduce a torsional force field for sp carbon to augment an in-plane atomistic potential of a previous work [G. Kalosakas et al., J.

Towards a formal definition of static and dynamic electronic correlations.

Some of the most spectacular failures of density-functional and Hartree-Fock theories are related to an incorrect description of the so-called static electron correlation. Motivated by recent progress in the N-representability problem of the one-body density matrix for pure states, we propose a method to quantify the static contribution to the electronic correlation. By studying several molecular systems we show that our proposal correlates well with our intuition of static and dynamic electron correlation.

Structural deformations of two-dimensional planar structures under uniaxial strain: the case of graphene.

In the present work, a method for the study of the structural deformations of two dimensional planar structures under uniaxial strain is presented. The method is based on molecular mechanics using the original stick and spiral model and a modified one which includes second nearest neighbor interactions for bond stretching. As we show, the method allows an accurate prediction of the structural deformations of any two dimensional planar structure as a function of strain, along any strain direction in the elastic regime, if structural deformations are known along specific strain directions, which are used to calculate the stick and spiral model parameters.

Conditions for Describing Triplet States in Reduced Density Matrix Functional Theory.

We consider necessary conditions for the one-body reduced density matrix (1RDM) to correspond to a triplet wave function of a two-electron system. The conditions concern the occupation numbers and are different for the high spin projections, Sz = ±1, and the Sz = 0 projection. Hence, they can be used to test if an approximate 1RDM functional yields the same energies for both projections.

Orbitals from local RDMFT: Are they Kohn-Sham or natural orbitals?

Recently, an approximate theoretical framework was introduced, called local reduced density matrix functional theory (local-RDMFT), where functionals of the one-body reduced density matrix (1-RDM) are minimized under the additional condition that the optimal orbitals satisfy a single electron Schrödinger equation with a local potential. In the present work, we focus on the character of these optimal orbitals. In particular, we compare orbitals obtained by local-RDMFT with those obtained with the full minimization (without the extra condition) by contrasting them against the exact NOs and orbitals from a density functional calculation using the local density approximation (LDA).

Graphene allotropes under extreme uniaxial strain: an ab initio theoretical study.

Using density functional theory calculations, we study the response of three representative graphene allotropes (two pentaheptites and octagraphene) as well as graphene, to uniaxial strain up to their fracture limit. Those allotropes can be seen as distorted graphene structures formed upon periodically arranged Stone-Walles transformations. We calculate their mechanical properties (Young's modulus, Poisson's ratio, speed of sound, ultimate tensile strength and the corresponding strain), and we describe the pathways of their fracture.

Generalized Pauli constraints in reduced density matrix functional theory.

Functionals of the one-body reduced density matrix (1-RDM) are routinely minimized under Coleman's ensemble N-representability conditions. Recently, the topic of pure-state N-representability conditions, also known as generalized Pauli constraints, received increased attention following the discovery of a systematic way to derive them for any number of electrons and any finite dimensionality of the Hilbert space. The target of this work is to assess the potential impact of the enforcement of the pure-state conditions on the results of reduced density-matrix functional theory calculations.

Quasi-particle energy spectra in local reduced density matrix functional theory.

Recently, we introduced [N. N. Lathiotakis, N.

Constraining density functional approximations to yield self-interaction free potentials.

Self-interactions (SIs) are a major problem in density functional approximations and the source of serious divergence from experimental results. Here, we propose to optimize density functional total energies in terms of the effective local potential, under constraints for the effective potential that guarantee it is free from SI errors and consequently asymptotically correct. More specifically, we constrain the Hartree, exchange and correlation potential to be the electrostatic potential of a non-negative effective repulsive density of N - 1 electrons.

Optical and magnetic properties of boron fullerenes.

We report linear response properties of the recently proposed boron fullerenes [N. Gonzalez Szwacki et al., Phys.