Time Dependent Density Functional Theory offers a good accuracy/computational cost ratio among different methods used to predict the electronic structure for molecules of practical interest. The Coupled Electronic Oscillator (CEO) formalism was recently shown to accurately predict Nonlinear Optical (NLO) properties of organic chromophores when combined with Time Dependent Density Functional Theory. Unfortunately, CEO does not lend itself easily to interpretation of the structure activity relationships of chromophores. On the other hand, the Sum Over States formalism in combination with semiempirical wavefunction methods has been used in the past for the design of simplified essential states models. These models can be applied to optimization of NLO properties of interest for applications. Unfortunately, TD-DFT can not be combined directly with SOS because state-to-state transition dipoles are not defined in the linear response TD approach. In this work, a second order CEO approach to TD-DFT is simplified so that properties of double excited states and state-to-state transition dipoles may be expressed through the combination of linear response properties. This approach is termed the a posteriori Tamm-Dancoff approximation (ATDA), and validated against high-level wavefunction theory methods. Sum over States (SOS) and related Two-Photon Transition Matrix formalism are then used to predict Two-Photon Absorption (2PA) profiles and anisotropy, as well as Second Harmonic Generation (SHG) properties. Numerical results for several conjugated molecules are in excellent agreement with CEO and finite field calculations, and reproduce experimental measurements well.

The ferromagnetic shape memory alloy, Fe7Pd3, not only offers promising applications, but also reveals a number of unresolved scientific questions, including coupling between a series of martensite and order–disorder transitions, which are in the focus of the present study. To address and understand these aspects, which are of particular importance for controlling phase stability in Fe7Pd3, an ab initio based Monte Carlo simulation code is developed, whose results demonstrate that equilibrated ordered or disordered phases show distinct dependencies coupled to temperature and lattice structure. Moreover, in equiatomic domains emerging from initially randomized disorder, an intermediate, entropy stabilized phase is identified with significantly higher magnetic anisotropy energy, being advantageous for miniaturized applications. This phase, among other observed configurations, is comprehensively characterized by free energy landscapes and magneto-structural coupling derived from vibrational analysis of molecular dynamics trajectories and full relativistic spin polarized density functional theory ground state calculations, respectively.

Doped IrTe2 is considered a platform for topological superconductivity and therefore receives currently a lot of interest. In addition, the superconductivity in these materials exists in close vicinity to electronic order and the formation of molecular orbital crystals, which we explore here by means of high-pressure single crystal x-ray diffraction in combination with density functional theory. Our crystallographic refinements provide detailed information about the structural evolution as a function of applied pressure up to 42 GPa. Using this structural information for density functional theory calculations, we show that the local multicenter bonding in IrTe2 is driven by changes in the Ir-Te-Ir bond angle. When the electronic order sets in, this bond angle decreases drastically, leading to a stabilization of a multicenter molecular orbital bond. This unusual local mechanism of bond formation in an itinerant material provides a natural explanation for the different electronic orders in IrTe2. It further illustrates the strong coupling of the electrons with the lattice and is most likely relevant for the superconductivity in this material.

<p dir="ltr">Computational materials science has emerged as a powerful technique to discover and develop new materials in past decades, primarily because accurate computational modeling can act as guidance before performing experiments that are expensive and time-consuming. However, modeling material behaviors across different scales of length and time poses a challenge, accentuating the importance of choosing appropriate levels of approximations and theories. First principles calculations based on density functional theory (DFT) are essential to predict the electronic structure of periodic crystalline systems. We will discuss a prediction of chemical doping induced metal-to-insulator transition (MIT) of transition metal perovskites owing to the variation of the electronic occupation. Nevertheless, electronic structure predictions based on DFT are not without limitation as it fails when treating strongly correlated electronic system due to the over-delocalization of valence electrons. In principle, adding on-site Hubbard U corrects this error with a low computational cost. Using an example of a two-dimensional rare-earth MXene, we demonstrate the essence of choosing the appropriate U value self-consistently for the prediction of electronic and magnetic configurations. Furthermore, molecular dynamics (MD) can be employed to study the dynamic evolution of complex condensed systems with thousands to millions of atoms at the atomistic and molecular levels. Carbon fiber manufacturing is an established industry, though the fiber produced achieves only 10% of its theoretical tensile strength. Therefore, optimizing the carbon fiber processing is a pressing topic. To achieve this, we study two steps, spinning and stabilization, of polyacrylonitrile (PAN)-based fiber fabrication at the molecular level using MD. We will discuss the realistic molecular structure of the spun PAN and the properties affected by its structural heterogeneity. Moreover, for the following step, we develop a PAN stabilization simulator, an automated workflow that addresses the underlying chemistry and the molecular-level structure-property relationship, often inaccessible through experiments.</p>

The reactivity and bonding of an ethinyl-functionalized cyclooctyne on Si(001) is studied by means of density functional theory. This system is promising for the organic functionalization of semiconductors. Singly bonded adsorption structures are obtained by [2 + 2] cycloaddition reactions of the cyclooctyne or ethinyl group with the Si(001) surface. A thermodynamic preference for adsorption with the cyclooctyne group in the on-top position is found and traced back to minimal structural deformation of the adsorbate and surface with the help of energy decomposition analysis for extended systems (pEDA). Starting from singly bonded structures, a plethora of reaction paths describing conformer changes and consecutive reactions with the surface are discussed. Strongly exothermic and exergonic reactions to doubly bonded structures are presented, while small reaction barriers highlight the high reactivity of the studied organic molecule on the Si(001) surface. Dynamic aspects of the competitive bonding of the functional groups are addressed by ab initio molecular dynamics calculations. Several trajectories for the doubly bonded structures are obtained in agreement with calculations using the nudged elastic band approach. However, our findings disagree with the experimental observations of selective adsorption by the cyclooctyne moiety, which is critically discussed.

En stor samling dimolekylära vätebindningar med formen A – H … B, där AH är en alkyn, alkohol eller tiol och B = [Br–, Cl–, NH3, HCN] beräknas och utvärderas med Kohn-Sham täthetsfunktionalteori tillsammans med bassetet m062x/6-311+g(2df.2p). Dessa komplex utvärderas även med en punktladdningsmodell (som använder samma metod och basset), där atomerna i vätebindningsmottagaren B byts ut mot laddningar som passats för att återskapa laddningsfördelningen runt molekylen, med målet att separera och isolera de elektrostatiska och polariserande energikomponenterna från de totala interaktionsenergierna. Med hjälp av detta tillvägagångssätt visade det sig att vätebindningars komplexeringsenergi (i.e. interaktionsenergin med energikostnaden för att deformera atomkärnornas rymdgeometri borttagen), oberoende av karaktären hos monomeren AH eller B, till stor del består av elektrostatik och polarisation, medan laddningsutbyte, dispersion, och andra resttermer endast utgör en liten del av den totala interaktionen. Fördelningen mellan elektrostatik och polarisation varierar beroende på typen av monomerer i vätebindningen, men deras summa, den resulterande punktladdningsenergin, korrelerar linjärt (ΔECompl = 0.85ΔEPC ) med R2 = 0.995 över energiomfånget 0 &lt; ΔECompl &lt; 50 kcal mol–1. Detta blir ännu mer anmärkningsvärt då inkluderingen av komplexeringsenergierna från halogenbindningar i samma korrelation inte förändrar korrelationskoefficienten avsevärt, vilket indikerar att båda bindningstyperna består av samma energikomponenter även då bindningarna i sig är väldigt olika. / A large set of dimeric hydrogen bonds of the type A – H … B, where AH is an alkyne, alcohol, or thiol and B = [Br–, Cl–, NH3, HCN]  are computed and evaluated using Kohn-Sham density functional theory together with the m062x/6-311+g(2df.2p) basis set. These complexes are also evaluated using a point charge (PC) approach (using the same method and basis set), where the atoms of the hydrogen bond acceptor B are substituted for charges that are optimized to reproduce the charge distribution of the molecule, with the purpose of separating and isolating the electrostatics- and polarization energy components of the interaction energies. Using this approach it was discovered that the complexation energy of hydrogen bonds (i.e.the interaction energy with the energy cost of nuclear deformation corrected for), independent on the nature of either monomer AH or B, are largely made up of electrostatics and polarization, while charge transfer, dispersion, and other rest terms only make up a small fraction of the total interaction. The composition of electrostatics and polarization vary depending on the type of monomers in the hydrogen bond, but their sum, the PC interaction energy, correlates linearly (ΔECompl = 0.85ΔEPC )  with R2 = 0.995 over an energy span of 0 &lt; ΔECompl &lt; 50 kcal mol–1. This is made even more remarkable by the inclusion of halogen bonded complexation energies in the same correlation without changing the correlation coefficient significantly, indicating that the two bond types are comprised of the same components even though they are remarkably different in origin.

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