Magnetic excitations and electronic interactions in Sr2_2CuTeO6_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 multi-reference 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 realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV.Comment: 10 pages, 7 figure

Orbital breathing effects in the computation of x-ray d-ion spectra in solids by ab initio wave-function-based methods

In existing theoretical approaches to core-level excitations of transition-metal ions in solids relaxation and polarization effects due to the inner core hole are often ignored or described phenomenologically. Here we set up an ab initio computational scheme that explicitly accounts for such physics in the calculation of x-ray absorption and resonant inelastic x-ray scattering spectra. Good agreement is found with experimental transition-metal $L$-edge data for the strongly correlated $d^9$ cuprate Li$_2$CuO$_2$, for which we determine the absolute scattering intensities. The newly developed methodology opens the way for the investigation of even more complex $d^n$ electronic structures of group VI B to VIII B correlated oxide compounds

Stability and Reversible Oxidation of Sub-Nanometric Cu5 Metal Clusters: Integrated Experimental Study and Theoretical Modeling**

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude “breathing” motions. A chemical phase diagram for Cu oxidation states of the Cu5-oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu

Stability of the HgS molecule and spectroscopy of its low lying electronic states

International audienceLarge scale Multireference Configuration Interactions (MRCI) and energy consistent relativistic pseudopotential (for the Hg atom) have been used to investigate the electronic structure, stability and spectroscopy of the low lying electronic states of the HgS molecule. The relative position of the two lowest electronic states, X-1 Sigma(+) and a(3)Pi, was found to be very sensitive to the quality of the basis set. Spin-orbit effects were taken into account leading to accurate spectroscopic data useful for the identification of the molecule. T-0 between the lowest components of the two states, X-1 Sigma(+)(0) and a(3)Pi(2), has been evaluated to be 0.142eV (3.5 kcal mol(-1)). Dipole moment functions were calculated for the lowest states; the rather large dipole moment of the X-1 Sigma(+) state makes possible the detection of vibrational transitions with a calculated ! e equal to 364cm(-1). Transitions between the X-1 Sigma(+) and the A(1)Pi states are predicted in the far IR domain with a T-0 = 5794cm(-1). The predissociation of the X-1 Sigma(+) and A(1)Pi states has been analysed and it has been shown that for the X-1 Sigma(+)(0) state only the vibrational levels below v = 11 are stable; higher levels are predissociated by the a(3)Pi(0) state. The effective dissociation energy of the X-1 Sigma(+)(0) state of HgS can thus be estimated to be 0.47eV (6.5 kcal mol(-1)). For the A(1)Pi state, the levels with v > 8 are predissociated by the dissociative b(3)Sigma(-) state

Ab initio ro-vibronic spectroscopy of the Π2 PCS radical and Σ+1PCS− anion

International audienceNear-equilibrium potential energy surfaces have been calculated for both the PCS radical and its anionusing a composite coupled cluster approach based on explicitly correlated F12 methods in order toprovide accurate structures and spectroscopic properties. These transient species are still unknownand the present study provides theoretical predictions of the radical and its anion for the first time.Since these species are strongly suggested to play an important role as intermediates in the interstellarmedium, the rotational and vibrational spectroscopic parameters are presented to help aid in theidentification and assignment of these spectra. The rotational constants produced will aid in groundbasedobservation. Both the PCS radical and the PCS– anion are linear. In the PCS– anion, which hasa predicted adiabatic electron binding energy (adiabatic electron affinity of PCS) of 65.6 kcal/mol,the P–C bond is stronger than the corresponding neutral radical showing almost triple bond character,while the C–S bond is weaker, showing almost single bond character in the anion. The PCS anionshows a smaller rotational constant than that of the neutral. The !3 stretching vibrational frequenciesof PCS are red-shifted from the radical, while the !1 and !2 vibrations are blue-shifted with !1demonstrating the largest blue shift. The ro-vibronic spectrum of the PCS radical has been accuratelycalculated in variational nuclear motion calculations including both Renner-Teller (RT) and spinorbit(SO) coupling effects using the composite potential energy near-equilibrium potential energyand coupled cluster dipole moment surfaces. The spectrum is predicted to be very complicated even atlowenergies due to the presence of a strong Fermi resonance between the bending mode and symmetricstretch, but also due to similar values of the bending frequency, RT, and SO splittings

Model estimates of inelastic calcium-hydrogen collision data for non-LTE stellar atmospheres modeling

Aims. Inelastic processes in low-energy Ca + H and Ca+ + H− collisions are treated for the states from the ground state up to the ionic state with the aim to provide rate coefficients needed for non-LTE modeling of Ca in cool stellar atmospheres. Methods. The electronic molecular structure was determined using a recently proposed model approach that is based on an asymptotic method. Nonadiabatic nuclear dynamics were treated by means of multichannel formulas, based on the Landau-Zener model for nonadiabatic transition probabilities. Results. The cross sections and rate coefficients for inelastic processes in Ca + H and Ca+ + H− collisions were calculated for all transitions between 17 low-lying covalent states plus the ionic state. It is shown that the highest rate coefficient values correspond to the excitation, de-excitation, ion-pair formation, and mutual neutralization processes involving the Ca(4s5s 1,3S) and the ionic Ca+ + H− states. The next group with the second highest rate coefficients includes the processes involving the Ca(4s5p 1,3P), Ca(4s4d 1,3D), and Ca(4s4p 1P) states. The processes from these two groups are likely to be important for non-LTE modeling

Communication: A combined periodic density functional and incremental wave-function-based approach for the dispersion-accounting time-resolved dynamics of He-4 nanodroplets on surfaces: He-4/graphene

In this work we propose a general strategy to calculate accurate He-surface interaction potentials. It extends the dispersionless density functional approach recently developed by Pernal et al. [Phys. Rev. Lett. 103, 263201 (2009)] to adsorbate-surface interactions by including periodic boundary conditions. We also introduce a scheme to parametrize the dispersion interaction by calculating two- and three-body dispersion terms at coupled cluster singles and doubles and perturbative triples (CCSD(T)) level via the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)]. The performance of the composite approach is tested on He-4/graphene by determining the energies of the low-lying selective adsorption states, finding an excellent agreement with the best available theoretical data. Second, the capability of the approach to describe dispersionless correlation effects realistically is used to extract dispersion effects in time-dependent density functional simulations on the collision of He-4 droplets with a single graphene sheet. It is found that dispersion effects play a key role in the fast spreading of the He-4 nanodroplet, the evaporation-like process of helium atoms, and the formation of solid-like helium structures. These characteristics are expected to be quite general and highly relevant to explain experimental measurements with the newly developed helium droplet mediated deposition technique. (C) 2014 AIP Publishing LLC

Ab initio ro-vibrational spectroscopy of the group 11 cyanides: CuCN, AgCN, and AuCN

Accurate near-equilibrium potential energy and dipole moment functions have been calculated for the linear coinage-metal cyanides CuCN, AgCN, and AuCN using coupled cluster methods and sequences of correlation consistent basis sets. The explicitly correlated CCSD(T)-F12b method is used for the potential energy surfaces (PESs) with inclusion of core correlation, and is combined with contributions from molecular spin-orbit coupling, scalar relativity, and effects due to higher order electron correlation. The resulting composite PESs are used in both perturbative and variational calculations of the ro-vibrational spectra. In addition to accurate equilibrium geometries, the ro-vibrational spectra are predicted, which are found to be relatively intense in the 200–600 cm−1 range due to the bending and metal–carbon stretching modes. The CN stretch near 2165 cm−1 is also predicted to carry enough intensity to allow its observation by experiment. A strong Fermi-resonance is predicted between the first overtone of the bend and the fundamental of the metal–carbon stretch for both CuCN and AgCN. The heats of formation at 0 K are predicted from their calculated atomization energies to be 89.8, 88.6, and 104.5 kcal mol−1 for CuCN, AgCN, and AuCN, respectively

Assessing the Performance of Dispersionless and Dispersion-accounting Methods: Helium Interaction with Cluster Models of the TiO2(110) Surface

18 pags.; 9 figs.; 4 tabs.As a prototypical dispersion-dominated physisorption problem, we analyze here the performance of dispersionless and dispersion-accounting methodologies on the helium interaction with cluster models of the TiO2(110) surface. A special focus has been given to the dispersionless density functional dlDF and the dlDF+Das construction for the total interaction energy (K. Pernal, R. Podeswa, K. Patkowski, and K. Szalewicz, Phys. Rev. Lett. 109 (2009) 263201), where Das is an effective inter-atomic pairwise functional form for the dispersion. Likewise, the performance of Symmetry-Adapted Perturbation Theory (SAPT) method is evaluated, where the interacting monomers are described by density functional theory (DFT) with the dlDF, PBE, and PBE0 functionals. Our benchmarks include CCSD(T)-F12b calculations and comparative analysis on the nuclear bound states supported by the He-cluster potentials. Moreover, intra- and inter-monomer correlation contributions to the physisorption interactionare analyzed through the method of increments (H. Stoll, J. Chem. Phys. 97 (1992) 8449) at CCSD(T) level of theory. This method is further applied in conjunction with a partitioning of the Hartree-Fock interaction energy to estimate individual interaction energy components, comparing them with those obtained using the different SAPT(DFT) approaches. The cluster size evolution of dispersionless and dispersion-accounting energy components is then discussed, revealing the reduced role of the dispersionless interaction and intra-monomer correlation when the extended nature of the surface is better accounted for. On the contrary, both post-Hartree-Fock and SAPT(DFT) results clearly demonstrate the high transferability character of the effective pairwise dispersion interaction whatever the cluster model is. Our contribution also illustrates how the method of increments can be used as a valuable tool not only to achieve the accuracy of CCSD(T) calculations using large cluster models, but also to evaluate the performance of SAPT(DFT) methods for the physically well-defined contributions to the total interaction energy. Overall, our work indicates the excellent performance of a dlDF+Das approach in which the parameters of the dispersion function are optimized using the smallest cluster model of the target surface. It also paves the way for further assessments of the dlDF+Das approach including periodic boundary conditions as a cost-efficient and accurate method to treat van der Waals adsorbate-surface interactions. © 2014 American Chemical SocietyThis work has been performed under Grants Nos. CCG08-CSIC/ESP-3680 from CSIC-CM and FIS2011-29596- C02-01 from DGI, Spain (FEDER). The support of COST Action CM1002 (CoDECS) is also gratefully acknowledged.Peer Reviewe