Ground-state Electronic Structure Research Articles

Stability of non-metal dopants to tune the photo-absorption of TiO2 at realistic temperatures and oxygen partial pressures: A hybrid DFT study

TiO2 anatase is considered to play a significant importance in energy and environmental research. However, for developing artificial photosynthesis with TiO2, the major drawback is its large bandgap of 3.2 eV. Several non-metals have been used experimentally for extending the TiO2 photo-absorption to the visible region of the spectrum. It’s therefore of paramount importance to provide theoretical guidance to experiment about the kind of defects that are thermodynamically stable at a realistic condition (e.g. Temperature (T), oxygen partial pressure ({{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}}), doping). However, disentangling the relative stability of different types of defects (viz. substitution, interstitial, etc.) as a function of charge state and realistic T, {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} is quite challenging. We report here using state-of-the-art first-principles based methodologies, the stability and meta-stability of different non-metal dopants X (X = N, C, S, Se) at various charge states and realistic conditions. The ground state electronic structure is very accurately calculated via density functional theory with hybrid functionals, whereas the finite T and {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} effects are captured by ab initio atomistic thermodynamics under harmonic approximations. On comparing the defect formation energies at a given T and {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} (relevant to the experiment), we have found that Se interstitial defect (with two hole trapped) is energetically most favored in the p-type region, whereas N substitution (with one electron trapped) is the most abundant defect in the n-type region to provide visible region photo-absorption in TiO2. Our finding validates that the most stable defects in X doped TiO2 are not the neutral defects but the charged defects. The extra stability of {({bf{S}}{bf{e}}{bf{O}})}_{{bf{O}}}^{+{bf{2}}} is carefully analyzed by comparing the individual effect of bond-making/breaking and the charge carrier trapping energies.

Origins of the Electronic Modulations of Bacterio- and Isobacteriodilactone Regioisomers.

Advances in the utilization of porphyrinoids for photomedicine, catalysis, and artificial photosynthesis require a fundamental understanding of the relationships between their molecular connectivity and resulting electronic structures. Herein, we analyze how the replacement of two pyrrolic Cβ═Cβ bonds of a porphyrin by two lactone (O═C-O) moieties modulates the ground-state thermodynamic stability and electronic structure of the resulting five possible pyrrole-modified porphyrin isomers. We made these determinations based on density functional theory (DFT) and time-dependent DFT computations of the optical spectra of all regioisomers. We also analyzed the computed magnetically induced currents of their aromatic π-systems. All regioisomers adopt the tautomeric state that maximizes aromaticity, whether or not transannular steric strains are incurred. In all isomers, the O═Cβ-Oβ bonds were found to support a macrocycle diatropic ring current. We attributed this to the delocalization of nonbonding electrons from the ring oxa- and oxo-atoms into the macrocycle. As a consequence of this delocalization, the dilactone regioisomers are as-or even more-aromatic than their hydroporphyrin congeners. The electronic structures follow different trends for the bacteriochlorin- and isobacteriochlorin-type isomers. The presence of either oxo- or oxa-oxygens conjugated with the macrocyclic π-system was found to be the minimal structural requirement for the regioisomers to exhibit distinct electronic properties. Our computational methods and mechanistic insights provide a basis for the systematic exploration of the physicochemical properties of porphyrinoids as a function of the number, relative orientation, and degree of macrocycle-π-conjugation of β-substituents, in general, and for dilactone-based porphyrinic chromophores, in particular.

Geometry Optimization Skills of DFTB: Simple to Complex Molecular Systems

As the Density-functional theory (DFT) is an exact theory in principle for computing ground state electronic structures of the multi-electron and many-body systems, it's approximate variants currently being used are far from fail-safe. One of the very fundamental problem that has become apparent is its inability to account the substantial effect of van der Waals (vdW) type interactions exist in large molecular assemblies. To overcome such problem, a density functional tight binding (DFTB) theory whose fundamental formulation is based on the DFT but implements Slater−Kirkwood model and Slater−Koster files with a focus on solid state systems having vdW interactions as a binding force has been widely used recently. Its self-consistent charge ( SCC) approach is more promising theoretical model due to introducing self-consistent calculation of Mulliken charges. Present work is aimed at evaluating the geometry optimization skills of such DFTB method while applying to very simple to quite complex molecular systems of the order: water, benzene, crystalline 1,4-bis (tri-methylsilyl) benzene, and crystalline siloxaalkane. We fully optimized the isolated molecules of each of them as well as the unit cell geometries of the last two specimens and measured the dimensions of the particular sets of bond lengths, bond angles, and torsional angles in each optimized geometry. These values are found to be in an excellent agreement with the concerned experimental values. It makes the DFTB method very versatile and superb quantum mechanical model for computing ground state electronic structures.

Nature of the excited state in lead iodide perovskite materials: Time-dependent charge density response and the role of the monovalent cation

Charge density response is responsible for the excited-state properties of lead iodide perovskites and is related to both the light absorption properties as well as subsequent electronic and lattice relaxation in the system, important for the working conditions of the material in solar cell applications. Here we investigate the nature of the excited state and its relation to pathways for electronic and lattice relaxations by performing time-dependent density-functional theory (TDDFT). Charge density response upon photoexcitation close to the band edge and deeper into the absorption spectra are investigated for three lead perovskite compounds with different $A$-site monovalent cations ${\mathrm{CsPbI}}_{3},\mathrm{C}{\mathrm{H}}_{2}{(\mathrm{N}{\mathrm{H}}_{2})}_{2}\mathrm{Pb}{\mathrm{I}}_{3}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.28em}{0ex}}(\mathrm{FAPb}{\mathrm{I}}_{3})$, and $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}(\mathrm{MAPb}{\mathrm{I}}_{3})$. The carrier cooling mechanism is analyzed and shows that the initial force acting on the nuclei follows the symmetry of the ground-state electronic structure upon photoexcitation with a force parallel to the polarization of the incoming light. This effect is investigated for the three different compounds and shows an initial force for induced ionic movement that depends on both the underlying symmetry of the inorganic lattice as well as on the type and orientation of the organic cation. The excess energy after thermalization under blue-light illumination is large enough for overcoming the activation energy for iodide migration and can thus trigger vacancy formation. Iodide vacancies are seen to be dipole-field compensated by the organic cation, with a shielding of the local field, and thus form an explanation for the defect tolerance found in these systems under photovoltaic operation. A partial charge transfer from the inorganic cage to the monovalent organic cation is predicted with TDDFT calculations for blue- and UV-light illumination with a population of antibinding orbitals in the N--H bond in both ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ (MA) and ${\mathrm{CH}}_{2}{({\mathrm{NH}}_{2})}_{2}$ (FA), where the implication for this is discussed in terms of the intrinsic photostability of organic cation containing lead perovskites. The results show the importance of a fundamental understanding of the excited-state properties of perovskite material to reveal the underlying mechanism for the defect tolerance and thus high photovoltaic performance when using organic dipolar cations as well as a rationale for using mixed halide perovskites to decrease the halide migration, effect of vacancy formation, and stability issues under blue- and UV-light illumination.

MS-CASPT2 Studies on the Photophysics of Selenium-Substituted Guanine Nucleobase.

The MS-CASPT2 method has been employed to optimize minimum-energy structures of 6-selenoguanine (6SeGua) and related two- and three-state intersection structures in and between the lowest five electronic states, i.e., S2(1ππ*), S1(1nπ*), T2(3nπ*), T1(3ππ*), and S0. In combination with MS-CASPT2 calculated linearly interpolated internal coordinate paths, the photophysical mechanism of 6SeGua has been proposed. The initially populated S2(1ππ*) state decays to either S1(1nπ*) or T2(3nπ*) states through a three-state S2/S1/T2 intersection point. The large S2/T2 spin–orbit coupling of 435 cm–1, according to the classical El-Sayed rule, benefits the S2 → T2 intersystem crossing process. The S1(1nπ*) state that stems from the S2 → S1 internal conversion process at the S2/S1/T2 intersection point can further jump to the T2(3nπ*) state through the S1 → T2 intersystem crossing process. This process does not comply with the El-Sayed rule, but it is still related to a comparatively large spin–orbit coupling of 39 cm–1 and is expected to occur relatively fast. Finally, the T2(3nπ*) state, which is populated from the above S2 → T2 and S1 → T2 intersystem crossing processes, decays to the T1(3ππ*) state via an internal conversion process. Because there is merely a small energy barrier of 0.11 eV separating the T1(3ππ*) minimum and an energetically allowed two-state T1/S0 intersection point, the T1(3ππ*) state still can decay to the S0 state quickly, which is also enhanced by a large T1/S0 spin–orbit coupling of 252 cm–1. Our proposed mechanism explains experimentally observed ultrafast intersystem crossing processes in 6SeGua and its 835-fold acceleration of the T1 state decay to the S0 state compared with 6tGua. Finally, we have found that the ground-state electronic structure of 6SeGua has more apparent multireference character.

Many-body perturbation theory calculations using the yambo code

yambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as Quantum ESPRESSO and Abinit. yambo’s capabilities include the calculation of linear response quantities (both independent-particle and including electron–hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron–phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools.

First principles investigation of Be3X2 (X = N, P, As) and their alloys for solar cell applications

The ground state electronic structure and absorption efficiency of α-Be3X2 (X = N, P or As) and their alloys are investigated using density functional theory. All pristine compounds and alloys are found to have direct band gaps at the zone center Γ point. The Be24PxN16-x and Be24AsxN16-x alloys (x = 4, 8, 12, or 16) are predicted to have suitable band gaps for solar absorber applications. The majority interband electronic transitions in the pristine compounds and the alloys are between the valence band p-states of N, P and As and the conduction band p-states of Be. The alloy systems show strong and wide absorption spectra, suggesting potential solar energy conversion applications.

Excited-State Potential Energy Surfaces, Conical Intersections, and Analytical Gradients from Ground-State Density Functional Theory.

Kohn-Sham density functional theory (KS-DFT) has been a well-established theoretical foundation for ground-state electronic structure and has achieved great success in practical calculations. Recently, utilizing the eigenvalues from KS or generalized KS (GKS) calculations as an approximation to the quasiparticle energies, our group demonstrated a method to calculate the excitation energies from (G)KS calculation on the ground-state ( N - 1)-electron system. This method is now called QE-DFT (quasiparticle energies from DFT). In this work, we extend this QE-DFT method to describe excited-state potential energy surfaces (PESs), conical intersections, and the analytical gradients of excited-state PESs. The analytical gradients were applied to perform geometry optimization for excited states. In conjunction with several commonly used density functional approximations, QE-DFT can yield PESs in the vicinity of the equilibrium structure with accuracy similar to that from time-dependent DFT (TD-DFT). Furthermore, it describes conical intersection well, in contrast to TD-DFT. Good results for geometry optimization, especially bond length, of low-lying excitations for 14 small molecules are presented. The capability of describing excited-state PESs, conical intersections, and analytical gradients from QE-DFT and its efficiency based on just ground-state DFT calculations should be of great interest for describing photochemical and photophysical processes in complex systems.

Development of interatomic potentials for the complex binary compound Sb2Te3 and the prediction of thermal conductivity

Molecular dynamics (MD) simulation has emerged as a powerful predictive tool to study mechanical and thermal properties of materials, without the requirement for any fitting parameter inputs such as phonon relaxation time. However, the lack of suitable interatomic potentials for many complex materials greatly prohibits effective use of MD simulations to investigate properties of bulk materials and nanostructures. In this paper, we use the method of fitting to an ab initio energy surface to develop interatomic potential parameters for the complex binary material antimony telluride, which has important applications in thermoelectric energy generation. Density-functional theory is used to calculate the ground-state electronic structure of the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ crystal, following which the total energies of a series of artificially distorted lattice configurations are calculated to create the energy surface. A Morse potential functional form is fitted to the energy surface and experimental data, and the parameters are used to calculate the bulk crystal properties and phonon spectra using lattice dynamics. Our parameters are able to reproduce the lattice structure, elastic constants, and acoustic phonon dispersion in good agreement with experimental data. MD simulations are performed using the fitted potential to calculate the thermal conductivity of bulk ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ using the Green-Kubo method. The predicted thermal conductivity shows a $1/T$ variation in both in-plane and cross-plane directions with the results in the range of experimental measurements. Frequency domain normal mode analysis is used to calculate the modal phonon relaxation times and the accumulation of thermal conductivity with respect to phonon mean free paths. The results show that phonons with mean free paths between 3 and 100 nm contribute to 80% of the total cross-plane thermal conductivity.

In silico and in vitro studies of transition metal complexes derived from curcumin–isoniazid Schiff base

A series of transition metal complexes have been synthesized from biologically active curcumin and isoniazid Schiff base. They are characterized by various spectral techniques like UV–Vis, Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) and mass spectroscopies. Moreover, elemental analysis, magnetic susceptibility and molar conductivity measurements are also carried out. All these data evidence that the metal complexes acquire square planar except zinc(II) which adopts a tetrahedral geometry, and they are non-electrolytic in nature. Groove mode of binding between the calf thymus DNA (CT DNA) and metal complexes is confirmed by electronic absorption titration, viscosity and cyclic voltammetry studies. In addition to that, all the metal complexes are able to cleave pUC 19 DNA. Optimized geometry and ground-state electronic structure calculations of all the synthesized compounds are established out by density functional theory (DFT) using B3LYP method which theoretically reveals that copper(II) complex explores higher stability and higher biological accessibility. This is experimentally corroborated by antimicrobial studies. In silico Absorption, Distribution, Metabolism, Excretion (ADME) studies reveal the biological potential of all synthesized complexes, and also biological activity of the ligand is predicted by PASS online biological activity prediction software. Molecular docking studies are also carried out to confirm the groove mode of binding and receptor–complex interactions.

Unusually Slow Internal Conversion in N-Heterocyclic Carbene/Carbanion Cyclometallated Ru(II) Complexes: A Hammett Relationship.

A series of six [Ru(bpy)2(NHC-R)]+ complexes were synthesized and characterized, where bpy = 2,2'-bipyridine and NHC-R is an N-heterocyclic carbene covalently linked to a carbanion with a number of substituents, R = -OMe (1), -Me (2), -H (3), -Cl (4), -CO2Et (5), and -NO2 (6). The effects of these strongly σ-donating NHC-R ligands on the ground-state electronic structure and on the excited-state character and dynamics were probed using electrochemistry, TD-DFT calculations, and steady-state absorption and emission spectroscopies, along with ultrafast transient absorption and time-resolved IR measurements. The excitation of 1-5 with a 400 nm pulse (irf = 85 fs) results in the population of a high energy singlet state, Sn, that rapidly intersystem crosses into a high-lying triplet state, Tn. Over the course of 7-22 ps, Tn relaxes to the lowest lying triplet state, T1, which is metal/ligand-to-ligand charge transfer, 3Ru(d)/NHC(π) → bpy(π*) in character. These 3ML-LCT states decay to regenerate the ground state with lifetimes, τ, that range from <8 to 15 ns at 298 K and from 10 to 23 ns at 77 K in CH3CN. Both the excited-state lifetime at 77 K and the Tn → T1 rate of internal conversion of 1-5 are dependent on the substituent R, and the latter correlates with the Hammett parameter (σ+p) of the NHC-R ligand. Excitation of 1-5 with low energy light, 550-670 nm, does not result in the population of Tn, as only T1 is observed. In the case of 6, excitation is expected to populate a 1Ru(d)/NHC(π) → NHC(π*) state localized on the NHC-NO2 ligand, which decays to a higher energy 3Ru(d)/NHC(π) → NHC(π*) state followed by internal conversion to the 3Ru(d)/NHC(π) → bpy(π*) T1 state with τ = 250 ps; the population of both states is independent of excitation wavelength in 6. This work demonstrates that the introduction of one NHC-R ligand in these complexes permits the population of a higher energy triplet state that decays to T1 in the picosecond time range. The relatively slow Tn → T1 internal conversion in these complexes makes the population of the higher-energy state potentially useful for more efficient charge injection into semiconductors for solar energy conversion or to aid in accessing dissociative metal-centered states for drug delivery. Overall, this work shows the ability to synthetically access valuable excited-state dynamics using the two different Ru-C bonds of the asymmetric NHC-R ligands.

Impact of metal ns2 lone pair on luminescence quantum efficiency in low-dimensional halide perovskites

Based on first-principles calculations, we show that chemically active metal ns2 lone pairs play an important role in exciton relaxation and dissociation in low-dimensional halide perovskites. We studied excited-state properties of several recently discovered luminescent all-inorganic and hybrid organic-inorganic zero-dimensional (0D) Sn and Pb halides. The results show that, despite the similarity in ground-state electronic structure between Sn and Pb halide perovskites, the chemically more active Sn2+ lone pair leads to stronger excited-state structural distortion and larger Stokes shift in Sn halides. The enhanced Stokes shift hinders excitation energy transport, which reduces energy loss to defects and increases the photoluminescence quantum efficiency (PLQE). The presence of the ns2 metal cations in the 0D halide perovskites also promotes the exciton dissociation into electron and hole polarons especially in all-inorganic compounds, in which the coupling between metal-halide clusters is significant.

Crystal and electronic structure studies on transparent conducting nitrides A3N2 (A = Mg, Zn and Sn) and Sn3N4

Finding potential materials for solar cell applications is essential to reduce cost and enhance efficiency. We have employed Density Functional Theory (DFT) based calculations for novel nitrides of type A3N2 (A = Mg, Zn ans Sn) and Sn3N4 to find the ground state crystal and electronic structure. The structural parameters optimized by theoretical calculation are in good agreement with the experimental parameters. In order to obtain Sn3N2 and Sn3N4 experimentally, we are suggesting a new synthesis route from the calculated enthalpy of formation. The studied nitrides exhibit semiconductor behavior with direct band gaps. Band gap of 1.7 eV is obtained for Mg3N2 from GGA calculation. For Sn3N4 band gaps of 0.24 eV and 0.7 eV are obtained using GGA and LDA calculations, respectively, whereas GGA+U was used to obtain band gap in Zn3N2. The bonding behavior is analyzed in detail by using charge density and electron localization function plots. In addition, COHP (Crystal Orbital Hamiltonian Population) is utilized to retrieve bond strength values. Charge transfer from cation to anion decreases from Mg to Sn and correspondingly bond strength between the metal and Nitrogen atoms is also found to increase from Mg to Sn, which indicates increase in covalent nature of bonding from Mg to Sn. Zn3N2 is found to have the possibility for n-type as well as p-type doping because of the low effective mass of electrons and holes compared to other studied nitrides. Hence, Zn3N2 has suitable conductive properties to be used as a solar cell material.

Quantum chemical studies of structures and spin Hamiltonian parameters of iron transferrin using isolated and embedded clusters models

Abstract Density functional theory (DFT) based calculations using large cluster models are used to elucidate the ground state electronic structure of iron bound transferrin. Explicit incorporation of second coordination amino acid residues and crystallographic water molecules anchors the active site. Our calculations clearly suggest that tyrosine amino acid (Tyr188) residue is bound to iron when the structures are optimized within the continuum solvation model. However, in the gas phase optimized structure, we note that Tyr188 is unbound to Fe (by more than 3 Å). The Mössbauer isomer shift \((\delta )\) and quadrupolar splitting \((\Delta \hbox {E}_{\mathrm{q}})\) of iron transferrin are in line with the experimental data only when Tyr188 is bound to Fe(III). Further, the computed oxygen hyperfine coupling constant value is very large (\(-14.5\) MHz) when bound to iron which can be verified through \(^{17}\hbox {O}\) NMR experiments. We propose that Tyr188 is strongly bound to Fe(III) at physiological pH, which needs to be protonated (acidic pH) to weaken this bond, thus the metal release pathway can be possible only in acidic conditions.Graphical Abstract Theoretical spectroscopic based calculations reveal that Tyr188 is bound to iron in transferrin at physiological pH.

First Principles Investigation of FeCo Alloy: Electronic and Optical Properties Study

The ground state electronic structures and optical properties of FeCo alloy have been reported using plane wave ultrasoft pseudopotential based on spin polarized density functional theory through first principles study. The crystallographic structure of FeCo consists with body-centered cubic lattice that is described in space group Im-3m (229). The geometry is optimized with zero applied pressure and the optimized lattice constant is found to be 2.854A. The electronic energy bands represent the overlapped between valence and conductance electronic states and confirm zero forbidden gaps i.e. metallic nature of the FeCo alloy. The Fermi surfaces manifest the anisotropic features of electronic energy dispersion along the high symmetry directions (X-R-M-G-R) of the Brillouin zone. The total density of states arises from the contribution of the electronic states of Co and Fe atoms. The calculated spin magnetic moments of FeCo alloy is 1.26μB. The spin magnetic moments mainly come from the exchange interactions among electronic spins, which confirms the strong electron-electron interactions. Moreover, the optical properties are computed which also attest the metallic behavior of the material. The optical measurements indicate that FeCo alloy is an optically anisotropic material. The obtained loss spectrum reveals the plasmonic excitations that is important for many magneto-optical applications.

Electronic structure of mono(Lewis base)-stabilized borylenes.

This work demonstrates that the mono(Lewis base)-stabilized three-center-six-electron (3c-6e) or LB → B-R type borylenes may have a ground state singlet (σ2π0 and σ0π2) or triplet (σ1π1) electronic structure, depending on the substitution group, Lewis base, and molecule topology (cyclic or acyclic). The singlet-triplet energy gap of a borylene increases with electron-donating substitution groups and decreases with Lewis bases of strong σ-donating and π-accepting feature (e.g., carbenes and pyridine). The gap can be widely varied (ranging up to about 300 kJ mol-1) and even inversed by the proper combination of substitution groups and Lewis bases. These borylenes may even have singlet-and-triplet coexisting ground state electronic structures, which imply their complex reactivity.

Ternary Copper (II) complex based chemical probes for DNA targeting: Cytotoxic activity under visible light

Among the bio‐metals, copper derivatives of O, N, S donor salicylaldehyde thiosemicarbazones have been obtained large interest due to their potential biological applications. Multisubstituted thiosemicarbazone ligand HL derived, new ternary Cu (II) complexes of [Cu(L)(bpy)] (1) and [Cu(L)(phen)] (2) (where, bpy is 2,2′‐bipyridine and phen is 1,10‐phenanthroline) have been synthesized and characterized using different physico‐chemical techniques. Complexes 1 and 2 are structurally characterized by single crystal X‐ray diffraction analysis, which reveals the trigonal bipyramidal distorted square based pyramid geometry of both the complexes 1 and 2 with ONS‐donor thiosemicarbazone bonded at the upper plane. The ground state electronic structures of complexes 1 and 2 have been investigated by using DFT/B3LYP theoretical evaluation with 6‐31G (d,p) and LANL2DZ basis set. The affinity towards DNA and human serum albumin has been evaluated using computational docking analysis and complex 2 exposes significant binding ability towards DNA and human serum albumin, because of its immense hydrophobicity. Consequently, complex 2 have higher antimicrobial in addition to the cytotoxic activity than complex 1 and free ligand HL under visible light. Along with, their apoptosis pathway of cytotoxicity has been evaluated by fluorescent microscopic analysis using acridine orange/ethidium bromide (AO/EB) stains. From these preliminary investigations, we believe that complex 2 can play a role as a more robust pharmacological agent.

Development of formalisms based on locally coupled open subsystems for calculations in molecular electronic structure and dynamics

Quantum embedding theories model a collection of interacting molecules as a set of subsystems, where each can be treated with a particular electronic structure method (wave function or density functional theory, for example); these theories can lead to computationally efficient and accurate algorithms. Motivated by challenges in the field, we previously described a formalism (that models two subsystems), which we call ``locally coupled open subsystems'' (LCOS), for the computation of ground-state energies, fractional electron-occupation numbers of the subsystems, and the size-consistent limit of subsystem dissociation. In this work we present the full (nonrelativistic) LCOS theory and the following extensions of our previous work: (i) the framework to study systems composed of multiple subsystems and a procedure to spin-adapt the auxiliary wave function that describes the partitioned system, so that its spin state matches that of the real system of interest; (ii) potential functionals and ideas to employ machine learning for the computation of ground-state densities and energies; (iii) formulation of two LCOS ground-state approaches where the fragments are assigned Kohn-Sham wave functions; and (iv) a time-dependent (TD) extension of these two ground-state formalisms in which the state of the subsystems evolves according to a unitary propagation; from this evolution we can extract TD electron populations of the fragments, for instance. We also discuss potential applications of the TD LCOS theory to linear photoabsorption and Raman spectroscopy. The developments presented in this work can lead to ground-state and TD electronic structure calculations where the computational scaling can be controlled, depending on the level of theory and the accuracy desired to model each one of the subsystems and their coupling.

Spherical aromaticity and electron delocalization in $${\text{C}}_8$$ C 8 and $${\text{B}}_4{\text{N}}_4$$ B 4 N 4 cubic systems

The electronic structure of the two isoelectronic species $${\text{C}}_8$$ and $${\text{B}}_4{\text{N}}_4$$ has been studied at several ab initio levels (Hartree–Fock, CASSCF, CASPT2, and coupled cluster). For both systems, the total position spread tensor and the electron entropy have been computed and compared. These quantities are indicators that give insight into the electron mobility (and, in the case of the spread, the behavior of different-spin electrons), and are a measure of the multi-reference character of an electronic wavefunction. Our results indicate that the two systems are deeply different. In fact, the $${\text{C}}_8$$ cluster shows a pronounced multi-reference character. The $${\text{B}}_4{\text{N}}_4$$ system, on the other hand, is very well described by a single reference wave function. Analysis of ground-state electronic structure unveils different electron delocalization behavior in the studied systems.

First-principles study of electron dynamics with explicit treatment of momentum dispersion on Si nanowires along different directions

ABSTRACTIn this research, ground-state electronic structure and optical properties along with photoinduced electron dynamics of Si nanowires oriented in various directions are reviewed. These nanowires are significant functional units of future nano-electronic devices. All observables are computed for a distribution of wave vectors at ambient temperature. Optical properties are computed under the approximation of momentum conservation. The total absorption is composed of partial contributions from fixed values of momentum. The on-the-fly non-adiabatic couplings obtained along the ab initio molecular dynamics nuclear trajectories are used as parameters for Redfield density matrix equation of motion. The main outcomes of this study are transition energies, light absorption spectra, electron and hole relaxation rates, and electron transport properties. The results of these calculations would contribute to the understanding of the mechanism of electron transfer process on the Si nanowires for optoelectronic applications.