Ag5-induced stabilization of multiple surface polarons on perfect and reduced TiO2 rutile (110).

The recent advent of cutting-edge experimental techniques allows for a precise synthesis of subnanometer metal clusters composed of just a few atoms, opening new possibilities for subnanometer science. In this work, via first-principles modeling, we show how the decoration of perfect and reduced TiO2 surfaces with Ag5 atomic clusters enables the stabilization of multiple surface polarons. Moreover, we predict that Ag5 clusters are capable of promoting defect-induced polarons transfer from the subsurface to the surface sites of reduced TiO2 samples. For both planar and pyramidal Ag5 clusters, and considering four different positions of bridging oxygen vacancies, we model up to 14 polaronic structures, leading to 134 polaronic states. About 71% of these configurations encompass coexisting surface polarons. The most stable states are associated with large inter-polaron distances (>7.5 Å on average), not only due to the repulsive interaction between trapped Ti3+ 3d1 electrons, but also due to the interference between their corresponding electronic polarization clouds [P. López-Caballero et al., J. Mater. Chem. A 8, 6842-6853 (2020)]. As a result, the most stable ferromagnetic and anti-ferromagnetic arrangements are energetically quasi-degenerate. However, as the average inter-polarons distance decreases, most (≥70%) of the polaronic configurations become ferromagnetic. The optical excitation of the midgap polaronic states with photon energy at the end of the visible region causes the enlargement of the polaronic wave function over the surface layer. The ability of Ag5 atomic clusters to stabilize multiple surface polarons and extend the optical response of TiO2 surfaces toward the visible region bears importance in improving their (photo-)catalytic properties and illustrates the potential of this new generation of subnanometer-sized materials.

Inelastic excitation and charge transfer processes for oxygen in collision with H atoms.

Potential energy functions of the OH molecule are investigated from small to large inter-atomic distances R. The electronic problem is treated using an efficient Full Configuration Interaction (Full CI) approach that avoids orbital jumps found usually in multi-configuration self-consistent-field followed by multi-reference configuration interaction calculations of excited states. The calculations are performed for all the doublet, quartet, and sextet OH molecular states, up to the O(2p34s 3S) + H(1s 2S) asymptote, and for the lowest O- + H+ and O+ + H- ionic states. Inter-atomic distances, ranging from 0.5 Å to 20 Å, are spanned with a very small step in order to describe accurately the avoided crossings between the adiabatic potential energy functions. The accuracy of the potentials at small and large R values is analyzed. These Full CI calculations provide for the first time a global description of the 40 lowest molecular states of OH, well suited for dynamical calculations. The resulting potentials are used to obtain first estimates of cross sections and rate coefficients for different inelastic processes through the multichannel approach. This method, based on a Landau-Zener formalism taking into account the ionic-covalent avoided crossings at large distances, gives reliable results for the most intense transitions. It is shown that the largest rate coefficients correspond to mutual neutralization and ion-pair production processes.

Crystal Field Splittings in Lanthanide Complexes: Inclusion of Correlation Effects beyond Second Order Perturbation Theory.

State-averaged complete active space self-consistent field (CASSCF) calculations and a subsequent spin-orbit calculation mixing the CASSCF wave functions (CASSCF/state-interaction with spin-orbit coupling) is the conventional approach used for ab initio calculations of crystal-field splittings and magnetic properties of lanthanide complexes. However, this approach neglects dynamical correlation. Complete active space second-order perturbation theory (CASPT2) can be used to account for dynamical correlation but suffers from the well-known problems of multireference perturbation theory, e.g., intruder state problems. Variational multireference configuration interaction (MRCI) calculations do not show these problems but are usually not feasible due to the large size of real lanthanide complexes. Here, we present a quasi-local projected internally contracted MRCI approach which makes MRCI calculations of lanthanide complexes feasible and allows assessing the influence of dynamical correlation beyond second-order perturbation theory. We apply the method to two well-studied molecules, namely, [Er{N(SiMe3)2}3] and {C(NH2)3}5[Er(CO3)4]·11H2O.

Calcium-hydrogen interactions for collisional excitation and charge transfer.

The accurate highly correlated ab initio calculations for ten low lying covalent Σ+2 states of CaH molecule, and one ionic Ca+H- state, are performed using large active space and extended basis set, with special attention to the long-range (6-20 Å) region where a number of avoided crossings between ionic and covalent states occur. These states are further transformed to a diabatic representation using a numerical diabatization scheme based on the minimization of derivative coupling. This results in a smooth diabatic Hamiltonian which can be easily fit to an analytic form. The diagonal elements of the diabatic potentials were then empirically corrected to reproduce experimental dissociation energies. Though the emphasis is on the asymptotic region, the obtained spectroscopic constants are in good agreement with available experimental and theoretical data. The resulting analytical Hamiltonian, after back transformation to adiabatic representation, is used to obtain cross sections for different inelastic processes using both the multichannel and the branching probability current approaches. It is shown that while for most intense transitions both approaches provide very close results, the multichannel approach underestimates the cross sections of weak transitions, as a consequence of the short-range avoided crossings that are accounted for only in the branching probability current method.

Magnetic Excitations and Electronic Interactions in Sr_{2}CuTeO_{6}: A Spin-1/2 Square Lattice Heisenberg Antiferromagnet.

Sr_{2}CuTeO_{6} presents an opportunity for exploring low-dimensional magnetism on a square lattice of S=1/2 Cu^{2+} ions. We employ ab initio multireference configuration interaction calculations to unravel the Cu^{2+} electronic structure and to evaluate exchange interactions in Sr_{2}CuTeO_{6}. The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr_{2}CuTeO_{6} is an almost ideal realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV.

Size-dependent mechanical properties of axial and radial mixed AlN/GaN nanostructure.

In this paper, continuum multiscale models are proposed to describe the size-dependent mechanical properties of two kinds of heterogeneous nanostructures: radially heterogeneous nanowires and longitudinally heterogeneous nanolaminates. In both cases, the continuum models involve additional surface/interface energies, which allow capturing size effects. Several models of imperfect interface models, like coherent and spring-layer ones, are shown to respectively capture the size effects, which are reported by first-principles calculations performed on heterogeneous nanostructures. In each case, a procedure is proposed to identify the parameters of the surface/interface model in the continuum framework, based on first-principles calculations performed on slab systems. The obtained continuum models allow avoiding full computations on atomistic models, which are not affordable for large sizes (diameters, layer thickness). An increase of the overall stiffness for both kinds of heterogeneous AlN/GaN nanostructures with the decrease of the dimensions is evidenced. The continuum models are then compared with full first-principles calculations to demonstrate their accuracy and their ability to capture size effects.

Towards an elastic model of wurtzite AlN nanowires.

Starting with ab initio calculations of AlN wurtzite [0001] nanowires with diameters up to 4 nm, a finite element method is developed to deal with larger nanostructures/nanoparticles. The ab initio calculations show that the structure of the nanowires can be well represented by an internal part with AlN bulk elastic properties, and one atomic surface layer with its own elastic behavior. The proposed finite element method includes surface elements with their own elastic properties using surface elastic coefficients deduced from the ab initio calculations. The elastic properties obtained with the finite element model compare very well with those obtained with the full ab initio calculations.

An optimized full-configuration-interaction nuclear orbital approach to a "hard-core" interaction problem: application to (3He)(N)-Cl2(B) clusters (N < or = 4).

An efficient full-configuration-interaction nuclear orbital treatment has been recently developed as a benchmark quantum-chemistry-like method to calculate ground and excited "solvent" energies and wave functions in small doped DeltaE(est) clusters (N < or = 4) [M. P. de Lara-Castells, G. Delgado-Barrio, P. Villarreal, and A. O. Mitrushchenkov, J. Chem. Phys. 125, 221101 (2006)]. Additional methodological and computational details of the implementation, which uses an iterative Jacobi-Davidson diagonalization algorithm to properly address the inherent "hard-core" He-He interaction problem, are described here. The convergence of total energies, average pair He-He interaction energies, and relevant one- and two-body properties upon increasing the angular part of the one-particle basis set (expanded in spherical harmonics) has been analyzed, considering Cl(2) as the dopant and a semiempirical model (T-shaped) He-Cl(2)(B) potential. Converged results are used to analyze global energetic and structural aspects as well as the configuration makeup of the wave functions, associated with the ground and low-lying "solvent" excited states. Our study reveals that besides the fermionic nature of (3)He atoms, key roles in determining total binding energies and wave-function structures are played by the strong repulsive core of the He-He potential as well as its very weak attractive region, the most stable arrangement somehow departing from the one of N He atoms equally spaced on equatorial "ring" around the dopant. The present results for N = 4 fermions indicates the structural "pairing" of two (3)He atoms at opposite sides on a broad "belt" around the dopant, executing a sort of asymmetric umbrella motion. This pairing is a compromise between maximizing the (3)He-(3)He and the He-dopant attractions, and suppressing at the same time the "hard-core" repulsion. Although the He-He attractive interaction is rather weak, its contribution to the total energy is found to scale as a power of three and it thus increasingly affects the pair density distributions as the cluster grows in size.

A full-configuration interaction "nuclear orbital" method to study doped 3HeN clusters (N< or =4).

An efficient full configuration interaction (FCI) treatment, based on the Jacobi-Davidson algorithm, is developed in order to study small doped (3)He(N) clusters. The state of each He atom in a given cluster is described by a set of wave-functions which by extention of the quantum-chemistry notation are caller here "nuclear orbitals". The FCI treatment is applied to the calculation of binding energies and helium natural orbitals of (3)He(N)...Br(2)(X) complexes. In agreement with our previous calculations using a Hartree-Fock approach [Phys. Rev. Lett. 93, 053401 (2004)], in which the He-He interaction is modified at small distances to account for short-range correlation effects, the lowest-energy states of each multiplet are found to be very close in energy. The natural orbital analysis, in turn, indicates the adequacy of the "nuclear orbital" approach in these systems.