Estimating Nonradiative Excited-State Lifetimes in Photoactive Semiconducting Nanostructures.

The time evolution of the exciton generated by light adsorption in a photocatalyst is an important feature that can be approached from full nonadiabatic molecular dynamics simulations. Here, a crucial parameter is the nonradiative recombination rate between the hole and the electron that form the exciton. In the present work, we explore the performance of a Fermi's golden rule-based approach on predicting the recombination rate in a set of photoactive titania nanostructures, relying solely on the coupling of the ground and first excited state. In this scheme the analysis of the first excited state is carried out by invoking Kasha's rule thus avoiding computationally expensive nonadiabatic molecular dynamics simulations and resulting in an affordable estimate of the recombination rate. Our results show that, compared to previous ones from nonadiabatic molecular dynamics simulations, semiquantitative recombination rates can be predicted for the smaller titania nanostructures, and qualitative values are obtained from the larger ones. The present scheme is expected to be useful in the field of computational heterogeneous photocatalysis whenever a complex and computationally expensive full nonadiabatic molecular dynamics cannot be carried out.

Artificial-intelligence-driven discovery of catalyst genes with application to CO2 activation on semiconductor oxides.

Catalytic-materials design requires predictive modeling of the interaction between catalyst and reactants. This is challenging due to the complexity and diversity of structure-property relationships across the chemical space. Here, we report a strategy for a rational design of catalytic materials using the artificial intelligence approach (AI) subgroup discovery. We identify catalyst genes (features) that correlate with mechanisms that trigger, facilitate, or hinder the activation of carbon dioxide (CO2) towards a chemical conversion. The AI model is trained on first-principles data for a broad family of oxides. We demonstrate that surfaces of experimentally identified good catalysts consistently exhibit combinations of genes resulting in a strong elongation of a C-O bond. The same combinations of genes also minimize the OCO-angle, the previously proposed indicator of activation, albeit under the constraint that the Sabatier principle is satisfied. Based on these findings, we propose a set of new promising catalyst materials for CO2 conversion.

Barnes Update Applied in the Gauss-Newton Method: An Improved Algorithm to Locate Bond Breaking Points.

A mechanochemical reaction is a reaction induced by mechanical energy. A general accepted model for this type of reaction consists of a first-order perturbation on the associated potential energy surface (PES) of the unperturbed molecular system due to mechanical stress or pulling force. Within this theoretical framework, the so-called optimal barrier breakdown points or optimal bond breaking points (BBPs) are critical points of the unperturbed PES where the Hessian matrix has a zero eigenvector that coincides with the gradient vector. Optimal BBPs are "catastrophe points" that are particularly important because their associated gradient indicates how to optimally harness tensile forces to induce reactions by transforming a chemical reaction into a barrierless process. Building on a previous method based on a nonlinear least-squares minimization to locate BBPs (Bofill et al., J. Chem. Phys. 2017, 147, 152710-10), we propose a new algorithm to locate BBPs of any molecular system based on the Gauss-Newton method combined with the Barnes update for a nonsymmetric Jacobian matrix, which is shown to be more appropriate than the Broyden update. The efficiency of the new method is demonstrated for a multidimensional model PES and two medium size molecular systems of interest in enzymatic catalysis and mechanochemistry.

Quantum equilibration of the double-proton transfer in a model system porphine.

There is a renewed interest in the derivation of statistical mechanics from the dynamics of closed quantum systems. A central part of this program is to understand how closed quantum systems, i.e., in the absence of a thermal bath, initialized far-from-equilibrium can share a dynamics that is typical to the relaxation towards thermal equilibrium. Equilibration dynamics has been traditionally studied with a focus on the so-called quenches of large-scale many-body systems. We consider here the equilibration of a two-dimensional molecular model system describing the double proton transfer reaction in porphine. Using numerical simulations, we show that equilibration indeed takes place very rapidly (∼200 fs) for initial states induced by pump-dump laser pulse control with energies well above the synchronous barrier. The resulting equilibration state is characterized by a strong delocalization of the probability density of the protons that can be explained, mechanistically, as the result of (i) an initial state consisting of a large superposition of vibrational states, and (ii) the presence of a very effective dephasing mechanism.

Investigating the character of excited states in TiO2 nanoparticles from topological descriptors: implications for photocatalysis.

Titanium dioxide (TiO2) nanoclusters (NCs) and nanoparticles (NPs) have been the focus of intense research in recent years since they play a prominent role in photocatalysis. In particular, the properties of their excited states determine the photocatalytic activity. Among the requirements for photocatalytic activity, low excitation energy and large separation of the charge carriers are crucial. While information regarding the first is straightforward from either experiment or theory, the information regarding the second is scarce or missing. In the present work we fill this gap through a topological analysis of the first singlet excited state of a series of TiO2 NCs, and anatase and rutile derived NPs containing up to 495 atoms. The excited states of all these systems in vacuo have been obtained from time-dependent density functional theory (TDDFT) calculations using hybrid functionals and the influence of water was taken into account through a continuum model. Three different topological descriptors based on the attachment/detachment one-electron charge density, are scrutinized: (i) charge transfer degree, (ii) charge density overlap, and (iii) distance between centroids of charge. The present analysis shows that the charge separation in the excited state strongly depends on the NP size and shape. The character of the electronic excitations, as arising from the analysis of the canonical Kohn-Sham molecular orbitals (MOs) or from natural transition orbitals (NTOs), is also investigated. The understanding and prediction of charge transfer and recombination in TiO2 nanostructures may have implications in the rational design of these systems to boost their photocatalytic potential.

Interplay between the Gentlest Ascent Dynamics Method and Conjugate Directions to Locate Transition States.

An algorithm to locate transition states on a potential energy surface (PES) is proposed and described. The technique is based on the GAD method where the gradient of the PES is projected into a given direction and also perpendicular to it. In the proposed method, named GAD-CD, the projection is not only applied to the gradient but also to the Hessian matrix. Then, the resulting Hessian matrix is block diagonal. The direction is updated according to the GAD method. Furthermore, to ensure stability and to avoid a high computational cost, a trust region technique is incorporated and the Hessian matrix is updated at each iteration. The performance of the algorithm in comparison with the standard ascent dynamics is discussed for a simple two dimensional model PES. Its efficiency for describing the reaction mechanisms involving small and medium size molecular systems is demonstrated for five molecular systems of interest.

Electronic Properties of Realistic Anatase TiO2 Nanoparticles from G0W0 Calculations on a Gaussian and Plane Waves Scheme.

The electronic properties of realistic (TiO2)n nanoparticles (NPs) with cuboctahedral and bipyramidal morphologies are investigated within the many-body perturbation theory (MBPT) G0W0 approximation using PBE and hybrid PBEx (12.5% Fock contribution) functionals as starting points. The use of a Gaussian and plane waves (GPW) scheme reduces the usual O4 computational time required in this type of calculation close to O3 and thus allows considering explicitly NPs with n up to 165. The analysis of the Kohn-Sham energy orbitals and quasiparticle (QP) energies shows that the optical energy gap (Ogap), the electronic energy gap (Egap), and the exciton binding energy (ΔEex) values decrease with increasing TiO2 NP size, in agreement with previous work. However, while bipyramidal NPs appear to reach the scalable regime already for n = 84, cuboctahedral NPs reach this regime only above n = 151. Relevant correlations are found and reported that will allow one to predict these electronic properties at the G0W0 level in even much larger NPs where these calculations are unaffordable. The present work provides a feasible and practical way to approach the electronic properties of rather large TiO2 NPs and thus constitutes a further step in the study of realistic nanoparticles of semiconducting oxides.

Theoretical Modeling of Electronic Excitations of Gas-Phase and Solvated TiO2 Nanoclusters and Nanoparticles of Interest in Photocatalysis.

The optical absorption spectra of (TiO2) n, nanoclusters ( n = 1-20) and nanoparticles ( n = 35, 84) have been calculated from the frequency-dependent dielectric function in the independent particle approximation under the framework of density functional theory. The PBE generalized gradient approach based functional, the so-called PBE+ U method and the PBE0 and PBEx hybrid functionals-containing 25% and 12.5% of nonlocal Fock exchange, respectively-have been used. The simulated spectra have been obtained in the gas phase and in water on previously PBE0 optimized atomic structures. The effect of the solvent has been accounted for by using an implicit water solvation model. For the smallest nanoclusters, the spectra show discrete peaks, whereas for the largest nanoclusters and for the nanoparticles they resemble a continuum absorption band. In the gas phase and for a given density functional, the onset of the absorption (optical gap, Ogap) remains relatively constant for all nanoparticle sizes although it increases with the percentage of nonlocal Fock exchange, as expected. For all tested functionals, the tendency of Ogap in water is very similar to that observed in the gas phase with an almost constant upshift. For comparison, the optical gap has also been calculated at the TD-DFT level with the PBEx functional in the gas phase and in water. Both approaches agree reasonably well although the TD-DFT gap values are lower than those derived from the dielectric-function. Overall, the position of the spectral maxima and the width of the spectra are relatively constant and independent of particle size which may have implications in the understanding of photocatalysis by TiO2.

Reliable and computationally affordable prediction of the energy gap of (TiO2)n (10 ≤ n ≤ 563) nanoparticles from density functional theory.

The optical gap (Ogap) of a set of (TiO2)n nanoclusters and nanoparticles with n = 10-563 and different morphologies such as spherical, octahedral, lamellar, or tubular finite structures is investigated based on a relativistic all-electron description along with a numerical atomic centered orbital basis set. Two different functionals are used, PBE and PBEx, the former corresponds to a standard implementation of the generalized gradient approximation (GGA) and the latter to a hybrid functional with 12.5% of Fock exchange which reproduces the band gap of bulk TiO2 anatase and rutile. It is shown that the inclusion of exchange Fock in the PBE functional promotes a systematic energy gap opening of 1.25 eV relative to the PBE values. Remarkably, a linear correlation is found between PBEx and PBE Ogap calculated values with concomitant similar correlations for the HOMO and LUMO orbital energies. However, it appears that PBEx induces a larger downshift on the HOMO orbital than the upshift observed on the LUMO one. The fact that the PBEx hybrid functional was shown to reproduce the experimental energy gaps of stoichiometric and reduced TiO2 bulk phases leads to a suitable and practical way to successfully estimate Ogap of TiO2 nanoparticles containing up to thousands of atoms with PBEx precision from computationally affordable PBE calculations.

Performance of the G0W0 Method in Predicting the Electronic Gap of TiO2 Nanoparticles.

Using a relativistic all-electron description and numerical atomic-centered orbital basis set, the performance of the G0W0 method on the electronic band gap of (TiO2)n nanoparticles (n = 1-20) is investigated. Results are presented for G0W0 on top of hybrid (PBE0 and a modified version with 12.5% of Fock exchange) functionals. The underestimation of the electronic band gap from Kohn-Sham orbital energies is corrected by the quasiparticle energies from the G0W0 method, which are consistent with the variational ΔSCF approach. A clear correlation between both methods exists regardless of the hybrid functional employed. In addition, the vertical ionization potential and electron affinity from quasiparticle energies show a systematic correlation with the ΔSCF calculated values. On the other hand, the shape of the nanoparticles promotes some deviations on the electronic band gap. In conclusion, this study shows the following: (i) A systematic correlation exists between band gaps, ionization potentials, and electron affinities of TiO2 nanoparticles as predicted from variational ΔSCF and G0W0 methods. (ii) The G0W0 approach can be successfully used to study the electronic band gap of realistic size nanoparticles at an affordable computational cost with a ΔSCF accuracy giving results that are directly related with those from photoemission spectroscopy. (iii) The quasiparticle energies are explicitly required to shed light on the photocatalytic properties of TiO2. (iv) The G0W0 approach emerges as an accurate method to investigate the photocatalytic properties of both nanoparticles and extended semiconductors.

Violation of DNA neighbor exclusion principle in RNA recognition.

DNA intercalation has been very useful for engineering DNA-based functional materials. It is generally expected that the intercalation phenomenon in RNA would be similar to that in DNA. Here we note that the neighbor-exclusion principle is violated in RNA by naphthalene-based cationic probes, in contrast to the fact that it is usually valid in DNA. All the intercalation structures are responsible for the fluorescence, where small naphthalene moieties are intercalated in between bases via π-π interactions. The structure is aided by hydrogen bonds between the cationic moieties and the ribose-phosphate backbone, which results in specific selectivity for RNA over DNA. This experimentally observed mechanism is supported by computationally reproducing the fluorescence and CD data. MD simulations confirm the unfolding of RNA due to the intercalation of probes. Elucidation of the mechanism of selective sensing for RNA over DNA would be highly beneficial for dynamical observation of RNA which is essential for studying its biological roles.

Spin Adapted versus Broken Symmetry Approaches in the Description of Magnetic Coupling in Heterodinuclear Complexes.

The performance of a series of wave function and density functional theory based methods in predicting the magnetic coupling constant of a family of heterodinuclear magnetic complexes has been studied. For the former, the accuracy is similar to other simple cases involving homodinuclear complexes, the main limitation being a sufficient inclusion of dynamical correlation effects. Nevertheless, these series of calculations provide an appropriate benchmark for density functional theory based methods. Here, the usual broken symmetry approach provides a convenient framework to predict the magnetic coupling constants but requires deriving the appropriate mapping. At variance with simple dinuclear complexes, spin projection based techniques cannot recover the corresponding (approximate) spin adapted solution. Present results also show that current implementation of spin flip techniques leads to unphysical results.

Intriguing Electrostatic Potential of CO: Negative Bond-ends and Positive Bond-cylindrical-surface.

The strong electronegativity of O dictates that the ground state of singlet CO has positively charged C and negatively charged O, in agreement with ab initio charge analysis, but in disagreement with the dipole direction. Though this unusual phenomenon has been fairly studied, the study of electrostatic potential (EP) for noncovalent interactions of CO is essential for better understanding. Here we illustrate that both C and O atom-ends show negative EP (where the C end gives more negative EP), favoring positively charged species, whereas the cylindrical surface of the CO bond shows positive EP, favoring negatively charged ones. This is demonstrated from the interactions of CO with Na(+), Cl(-), H2O, CO and benzene. It can be explained by the quadrupole driven electrostatic nature of CO (like N2) with very weak dipole moment. The EP is properly described by the tripole model taking into account the electrostatic multipole moments, which has a large negative charge at a certain distance protruded from C, a large positive charge on C, and a small negative charge on O. We also discuss the EP of the first excited triplet CO.

A molecular view of cisplatin's mode of action: interplay with DNA bases and acquired resistance.

The interaction of the widely used anticancer drug cisplatin with DNA bases was studied by EXAFS and vibrational spectroscopy (FTIR, Raman and INS), coupled with DFT/plane-wave calculations. Detailed information was obtained on the local atomic structure around the Pt(ii) centre, both in the cisplatin-purine (adenine and guanine) and cisplatin-glutathione adducts. Simultaneous neutron and Raman scattering experiments allowed us to obtain a reliable and definite picture of this cisplatin interplay with its main pharmacological target (DNA), at the molecular level. The vibrational experimental spectra were fully assigned in the light of the calculated pattern for the most favoured geometry of each drug-purine adduct, and cisplatin's preference for guanine (G) relative to adenine (A) within the DNA double helix was experimentally verified: a complete N by S substitution in the metal coordination sphere was only observed for [cDDP-A2], reflecting a somewhat weaker Pt-A binding relative to Pt-G. The role of glutathione on the drug's pharmacokinetics, as well as on the stability of platinated DNA adducts, was evaluated as this is the basis for glutathione-mediated intracellular drug scavenging and in vivo resistance to Pt-based anticancer drugs. Spectroscopic evidence of the metal's preference for glutathione's sulfur over purine's nitrogen binding sites was gathered, at least two sulfur atoms being detected in platinum's first coordination sphere.

An inelastic neutron scattering study of dietary phenolic acids.

The conformational preferences and hydrogen-bonding motifs of several potential chemopreventive hydroxycinnamic derivatives were determined by inelastic neutron scattering spectroscopy. The aim is to understand their recognized beneficial activity and establish reliable structure-activity relationships for these types of dietary phytochemicals. A series of phenolic acids with different hydroxyl/methoxyl ring substitution patterns were studied: trans-cinnamic, p-coumaric, m-coumaric, trans-caffeic and ferulic acids. Their INS spectra were completely assigned by theoretical calculations performed at the Density Functional Theory level, for the isolated molecule, dimeric centrosymmetric species and the solid (using plane-wave expansion approaches). Access to the low energy vibrational region of the spectra enabled the identification of particular modes associated with intermolecular hydrogen-bonding interactions, which are the determinants of the main conformational preferences and antioxidant capacity of these systems.

Polymorphism in cisplatin anticancer drug.

This study reports a combined experimental and theoretical study of the solid-state polymorphism of the anticancer agent cisplatin. A complete assignment was performed for the inelastic neutron scattering (INS) and Raman spectra collected simultaneously for cisplatin, at different temperatures, with a view to obtain reliable and definitive evidence of the relative thermal stability of its α and ß polymorphic species. A marked temperature-dependent hysteresis was observed, as previously reported in the literature. Theoretical calculations were carried out at the density functional theory level, using a plane-wave basis set approach and pseudopotentials. A detailed comparison with the experimental Raman and INS data showed that the α polymorph is present at the lowest temperatures, whereas the ß form occurs near room temperature. Furthermore, regions of coexistence of both forms are identified, which depend on the working mode (heating or cooling). These findings imply that Raman spectroscopy allows clear identification of the α and ß polymorphs at a given temperature and can unambiguously discriminate between them. Elucidation of the polymorphic equilibrium of this widely used anticancer drug is paramount for its pharmaceutical preparation and storage conditions.

Photochemistry in a dense manifold of electronic states: photodissociation of CH2ClBr.

We report electronically nonadiabatic dynamics calculations including spin-orbit coupling for the photodissociation of CH(2)ClBr to yield Cl((2)P(3∕2)), Cl((2)P(1∕2)), Br((2)P(3∕2)), and Br((2)P(1∕2)). The potential energy is a 24 × 24 matrix (divided up here into four 6 × 6 blocks in a first approximation to the problem), in a spin-coupled fully diabatic representation obtained by combining the spin-free fourfold way with single-center spin-orbit coupling constants. The spin-free calculations are carried out by multiconfiguration quasidegenerate perturbation theory, and the fully diabatic potentials including spin-orbit coupling are fit to a matrix reactive force field. The dynamics are carried out by the coherent switches with decay of mixing method in the diabatic representation. The results show qualitative agreement with experiment.

Quantitative integral cross sections for the H + CO2 → OH + CO reaction from a density functional theory-based potential energy surface.

The OH + CO → H + CO(2) reaction is important in combustion, atmospheric, and interstellar chemistry. Whereas the direct reaction has been extensively studied both experimentally and theoretically, the reverse reaction has received relatively less attention. Here we carry out a quasiclassical trajectory study of the hyperthermal H + CO(2)→ OH + CO reaction on a new interpolated potential energy surface based on the M06-2X density functional. The results reveal for the first time quantitative agreement with experiment for the reaction cross sections in the range of relative translational energies 1.2-2.5 eV. We attribute this excellent agreement to both the quality of the M06-2X energies, which closely reproduce CCSD(T) energies, and to the potential surface construction strategy that emphasizes both the direct and reverse reactions.

Verdict: Time-Dependent Density Functional Theory "Not Guilty" of Large Errors for Cyanines.

We assess the accuracy of eight Minnesota density functionals (M05 through M08-SO) and two others (PBE and PBE0) for the prediction of electronic excitation energies of a family of four cyanine dyes. We find that time-dependent density functional theory (TDDFT) with the five most recent of these functionals (from M06-HF through M08-SO) is able to predict excitation energies for cyanine dyes within 0.10-0.36 eV accuracy with respect to the most accurate available Quantum Monte Carlo calculations, providing a comparable accuracy to the latest generation of CASPT2 calculations, which have errors of 0.16-0.34 eV. Therefore previous conclusions that TDDFT cannot treat cyanine dyes reasonably accurately must be revised.

Density functional study of multiplicity-changing valence and Rydberg excitations of p-block elements: delta self-consistent field, collinear spin-flip time-dependent density functional theory (DFT), and conventional time-dependent DFT.

A database containing 17 multiplicity-changing valence and Rydberg excitation energies of p-block elements is used to test the performance of density functional theory (DFT) with approximate density functionals for calculating relative energies of spin states. We consider only systems where both the low-spin and high-spin state are well described by a single Slater determinant, thereby avoiding complications due to broken-symmetry solutions. Because the excitations studied involve a spin change, they require a balanced treatment of exchange and correlation, thus providing a hard test for approximate density functionals. We test three formalisms for predicting the multiplicity-changing transition energies. First is the ΔSCF method; we also test time-dependent density functional theory (TDDFT), both in its conventional form starting from the low-spin state and in its collinear spin-flip form starting from the high-spin state. Very diffuse basis functions are needed to give a qualitatively correct description of the Rydberg excitations. The scalar relativistic effect needs to be considered when quantitative results are desired, and we include it in the comparisons. With the ΔSCF method, most of the tested functionals give mean unsigned errors (MUEs) larger than 6 kcal/mol for valence excitations and MUEs larger than 3 kcal/mol for Rydberg excitations, but the performance for the Rydberg states is much better than can be obtained with time-dependent DFT. It is surprising to see that the long-range corrected functionals, which have 100% Hartree-Fock exchange at large inter-electronic distance, do not improve the performance for Rydberg excitations. Among all tested density functionals, ΔSCF calculations with the O3LYP, M08-HX, and OLYP functionals give the best overall performance for both valence and Rydberg excitations, with MUEs of 2.1, 2.6, and 2.7 kcal/mol, respectively. This is very encouraging since the MUE of the CCSD(T) coupled cluster method with quintuple zeta basis sets is 2.0 kcal/mol; however, caution is advised since many popular density functionals give poor results, and there can be very significant differences between the ΔSCF predictions and those from TDDFT.