Fernando D. Vila

Parameter-free calculations of X-ray spectra with FEFF9

We briefly review our implementation of the real-space Greens function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye-Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.

Scientific Computing in the Cloud

Large, virtualized pools of computational resources raise the possibility of a new, advantageous computing paradigm for scientific research. To help achieve this, new tools make the cloud platform behave virtually like a local homogeneous computer cluster, giving users access to high-performance clusters without requiring them to purchase or maintain sophisticated hardware.

Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network

We report autodetachment spectra of the mass-selected, anionic water clusters, (H2O)n−, n=2, 3, 5–9, 11 in the OH stretching region (3000–4000 cm−1), and interpret the spectra with the aid of ab initio calculations. For n⩾5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm−1, which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the n=5–11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear “chainlike” (H2O)n− species. We test this assignment by recording the spectral pattern of the cooled (argon solvated) HDO⋅(D2O)5− isotopomer over the entire OH stretching frequency range.

BINDING ENERGY OF THE RING FORM OF (H2O)6 : COMPARISON OF THE PREDICTIONS OF CONVENTIONAL AND LOCALIZED-ORBITAL MP2 CALCULATIONS

The binding energy of the ring form of (H2O)6 is calculated by means of the MP2 and localized‐orbital MP2 (LMP2) methods. The LMP2 method is found to be effective at reducing basis set superposition error in the electron correlation contribution to the binding energy. The inclusion of f and g functions on the O atoms and d and f functions on the H atoms leads to an increase of about 3.2 kcal/mol in the stability of the ring form of (H2O)6. Our best estimate of the binding energy is −44.3 kcal/mol. Of this, three‐body interactions contribute −11.60 kcal/mol, and the four‐ , five‐ , and six‐body interactions combined contribute −2.0 kcal/mol. Although inclusion of electron correlation energy is crucial for obtaining an accurate value of the two‐body interactions, the net effect of electron correlation on the three‐ and higher‐body interactions is only about 0.2 kcal/mol. Based on these results, a computationally efficient strategy for obtaining accurate binding energies of hydrogen‐bonded clusters is proposed.

Real-time time-dependent density functional theory approach for frequency-dependent nonlinear optical response in photonic molecules

We present ab initio calculations of frequency-dependent linear and nonlinear optical responses based on real-time time-dependent density functional theory for arbitrary photonic molecules. This approach is based on an extension of an approach previously implemented for a linear response using the electronic structure program SIESTA. Instead of calculating excited quantum states, which can be a bottleneck in frequency-space calculations, the response of large molecular systems to time-varying electric fields is calculated in real time. This method is based on the finite field approach generalized to the dynamic case. To speed the nonlinear calculations, our approach uses Gaussian enveloped quasimonochromatic external fields. We thereby obtain the frequency-dependent second harmonic generation beta(-2omega;omega,omega), the dc nonlinear rectification beta(0;-omega,omega), and the electro-optic effect beta(-omega;omega,0). The method is applied to nanoscale photonic nonlinear optical molecules, including p-nitroaniline and the FTC chromophore, i.e., 2-[3-Cyano-4-(2-{5-[2-(4-diethylamino-phenyl)-vinyl]-thiophen-2-yl}-vinyl)-5,5-dimethyl-5H-furan-2-ylidene]-malononitrile, and yields results in good agreement with experiment.

Accuracy of generalized gradient approximation functionals for density-functional perturbation theory calculations

We assess the validity of various exchange-correlation functionals for computing the structural, vibrational, dielectric, and thermodynamical properties of materials in the framework of density-functional perturbation theory (DFPT). We consider five generalized-gradient approximation (GGA) functionals (PBE, PBEsol, WC, AM05, and HTBS) as well as the local density approximation (LDA) functional. We investigate a wide variety of materials including a semiconductor (silicon), a metal (copper), and various insulators (SiO2 α-quartz and stishovite, ZrSiO4 zircon, and MgO periclase). For the structural properties, we find that PBEsol and WC are the closest to the experiments and AM05 performs only slightly worse. All three functionals actually improve over LDA and PBE in contrast with HTBS, which is shown to fail dramatically for α-quartz. For the vibrational and thermodynamical properties, LDA performs surprisingly very well. In the majority of the test cases, it outperforms PBE significantly and also the WC, PBEsol and AM05 functionals though by a smaller margin (and to the detriment of structural parameters). On the other hand, HTBS performs also poorly for vibrational quantities. For the dielectric properties, none of the functionals can be put forward. They all (i) fail to reproduce the electronic dielectric constant due to the well-known band gap problem and (ii) tend to overestimate the oscillator strengths (and hence the static dielectric constant).

Theoretical X-Ray Absorption Debye-Waller Factors

An approach is presented for theoretical calculations of the Debye-Waller factors in x-ray absorption spectra. These factors are represented in terms of the cumulant expansion up to third order. They account respectively for the net thermal expansion � (1) (T), the mean-square relative displacements � 2 (T), and the asymmetry of the pair distribution function � (3) (T). Similarly, we obtain Debye-Waller factors for x-ray and neutron scattering in terms of the mean-square vibrational amplitudes u 2 (T). Our method is based on density functional theory calculations of the dynamical matrix, together with an efficient Lanczos algorithm for projected phonon spectra within the quasiharmonic approximation. Due to anharmonicity in the interatomic forces, the results are highly sensitive to variations in the equilibrium lattice constants, and hence to the choice of exchangecorrelation potential. In order to treat this sensitivity, we introduce two prescriptions: one based on the local density approximation, and a second based on a modified generalized gradient approximation. Illustrative results for the leading cumulants are presented for several materials and compared with experiment and with correlated Einstein and Debye models. We also obtain Born-von Karman parameters and corrections due to perpendicular vibrations.

Basis set effects on the hyperpolarizability of CHCl3: Gaussian-type orbitals, numerical basis sets and real-space grids

Calculations of the hyperpolarizability are typically much more difficult to converge with basis set size than the linear polarizability. In order to understand these convergence issues and hence obtain accurate ab initio values, we compare calculations of the static hyperpolarizability of the gas-phase chloroform molecule (CHCl(3)) using three different kinds of basis sets: Gaussian-type orbitals, numerical basis sets, and real-space grids. Although all of these methods can yield similar results, surprisingly large, diffuse basis sets are needed to achieve convergence to comparable values. These results are interpreted in terms of local polarizability and hyperpolarizability densities. We find that the hyperpolarizability is very sensitive to the molecular structure, and we also assess the significance of vibrational contributions and frequency dispersion.

Theoretical optical and x-ray spectra of liquid and solid H2O

Theoretical optical and x-ray spectra of model structures of water and ice are calculated using a many-body perturbation theory, Bethe-Salpeter equation (BSE) approach implemented in the valence- and core-excitation codes ai2nbse and ocean. These codes use ab initio density functional theory wave functions from a plane-wave, pseudopotential code, quasiparticle self-energy corrections, and a BSE treatment of particle-hole interactions. This approach improves upon independent-particle methods through the inclusion of a complex, energy-dependent self-energy and screened particle-hole interactions to account for inelastic losses and excitonic effects. These many-body effects are found to be crucial for quantitative calculations of ice and water spectra.

Local electronic structure of dicarba-closo-dodecarboranes C2B10H12.

We report nonresonant inelastic X-ray scattering (NRIXS) measurement of core-shell excitations from both B 1s and C 1s initial states in all three isomers of the dicarba-closo-dodecarboranes C2B10H12. First, these data yield an experimental determination of the angular-momentum-projected final local density of states (l-DOS). We find low-energy resonances with distinctive local s- or p-type character, providing a more complete experimental characterization of bond hybridization than is available from dipole-transition limited techniques, such as X-ray absorption spectroscopies. This analysis is supported by independent density functional theory and real-space full multiple scattering calculation of the l-DOS which yield a clear distinction between tangential and radial contributions. Second, we investigate the isomer sensitivity of the NRIXS signal and compare and contrast these results with prior electron energy loss spectroscopy measurements. This work establishes NRIXS as a valuable tool for borane chemistry, not only for the unique spectroscopic capabilities of the technique but also through its compatibility with future studies in solution or in high-pressure environments. In addition, this work also establishes the real-space full multiple scattering approach as a useful alternative to traditional approaches for excited states calculations of aromatic polyhedral boranes and related systems.