Infrared photodissociation spectroscopy of (AlO)FeO: influence of Fe-substitution on small alumina clusters.

The infrared photodissociation spectra of He-tagged (AlO)FeO ( = 2-5), are reported in the Al-O and Fe-O stretching and bending spectral region (430-1200 cm) and assigned based on calculated harmonic IR spectra from density functional theory (DFT). The substitution of Fe for an Al center occurs preferentially at 3-fold oxygen coordination sites located at the cluster rim and with the Fe atom in the +III oxidation state. The accompanying elongation of metal oxygen bonds leaves the Al-O network structure nearly unperturbed (isomorphous substitution).

Reduction of Hartree-Fock Wavefunctions to Kohn-Sham Effective Potentials Using Multiresolution Analysis.

We present a highly accurate numerical implementation for computing the Kohn-Sham effective potentials for molecules based on a Hartree-Fock wavefunction and density, following the RKS approach of Staroverov and co-workers [ 2014, 140, 18A535]. Potentials and orbitals are represented in a multiresolution wavelet basis, avoiding basis set incompleteness-related issues. Together with the RKS method, the often occurring problems of oscillating potentials are removed.

Valence and Structure Isomerism of AlFeO: Synergy of Spectroscopy and Quantum Chemistry.

We provide spectroscopic and computational evidence for a substantial change in structure and gas phase reactivity of AlO upon Fe-substitution, which is correctly predicted by multireference (MR) wave function calculations. AlO exhibits a cone-like structure with a central trivalent O atom (C symmetry). The replacement of the Al- by an Fe atom leads to a planar bicyclic frame with a terminal Al-O radical site, accompanied by a change from the Fe/O to the Fe/O valence state.

Thermochemistry of FeOH Species: Assessment of Some DFT Functionals.

Thermochemical data for 20 anionic, cationic, and neutral gas-phase species, including Fe, FeO, FeOH, FeO, OFeOH, Fe(OH), Fe(HO), and Fe(HO) with oxidation states between +I and +IV for Fe and -I and -II for O, compiled by Schröder [ 2008, 112, 13215], are used to assess the performance of the "Jacob's ladder" functionals PBE, TPSS, PBE0, and TPSSh for the SVP, TZVP, and QZVP basis sets. In addition, the BP86 and B3LYP functionals are considered. The TPSSh functional performs best.

Direct determination of optimal pair-natural orbitals in a real-space representation: The second-order Moller-Plesset energy.

An efficient representation of molecular correlated wave functions is proposed, which features regularization of the Coulomb electron-electron singularities via the F12-style explicit correlation and a pair-natural orbital factorization of the correlation components of the wave function expressed in the real space. The pair-natural orbitals are expressed in an adaptive multiresolution basis and computed directly by iterative variational optimization. The approach is demonstrated by computing the second-order Moller-Plesset energies of small- and medium-sized molecules.

Temperature- and Structure-Dependent Optical Properties and Photophysics of BODIPY Dyes.

We report on the temperature- and structural-dependent optical properties and photophysics of a set of boron dipyrromethene (BODIPY) dyes with different substitution patterns of their meso-aryl subunit. Single-crystal X-ray diffraction analysis of the compounds enabled a classification of the dyes into a sterically hindered and a unhindered group. The steric hindrance refers to a blocked rotational motion of the aryl subunit around the bond connecting this moiety to the meso-position of the BODIPY core.

Evaluating the Solvent Stark Effect from Temperature-Dependent Solvatochromic Shifts of Anthracene.

The solvent Stark effect on the spectral shifts of anthracene is studied with temperature-dependent solvatochromic measurements. The Stark contribution Δv to the absorption shift Δv in polar solvents is measured to be Δv =(53±35) cm , in reasonable agreement with dielectric continuum theory estimate of 28 cm , whereas the major shift Δv ∼300 cm presumably originates from the solute quadrupole. We pay attention to the accurate correction of Δv for the nonpolar contribution that is crucial when the shifts are modest in magnitude.

Chemically accurate adsorption energies for methane and ethane monolayers on the MgO(001) surface.

A hybrid QM:QM method that combines MP2 as high-level method on cluster models with density functional theory (PBE+D2) as low-level method on periodic models is applied to adsorption of methane and ethane on the MgO(001) surface for which reliable experimental desorption enthalpies are available. Two coverages are considered, monolayer (every second Mg2+ ion occupied) and one quarter coverage (one of eight Mg2+ ions occupied). Structure optimizations are performed at the hybrid MP2:(PBE+D2) level, with the MP2 energies and forces counterpoise corrected for basis set superposition error and extrapolated to the complete basis set limit.

Coupled-Cluster in Real Space. 1. CC2 Ground State Energies Using Multiresolution Analysis.

A framework to calculate CC2 approximated coupled-cluster ground state correlation energies in a multiresolution basis is derived and implemented into the MADNESS library. The CC2 working equations are formulated in first quantization which makes them suitable for real-space methods. The first quantized equations can be interpreted diagrammatically using the usual diagrams from second quantization with adjusted interpretation rules.

Coupled-Cluster in Real Space. 2. CC2 Excited States Using Multiresolution Analysis.

We report a first quantized approach to calculate approximate coupled-cluster singles and doubles CC2 excitation energies in real space. The cluster functions are directly represented on an adaptive grid using multiresolution analysis. Virtual orbitals are neither calculated nor needed in this approach.

Analytic second nuclear derivatives of Hartree-Fock and DFT using multi-resolution analysis.

We present the formalism, implementation, and numerical results for the computation of second derivatives with respect to nuclear displacements of molecules in the formalism of multi-resolution analysis. The highly singular nuclear potentials are partially regularized to improve the numerical stability. Vibrational frequencies are well reproduced to within an RMS of a few cm compared to large basis set LCAO (linear combination of atomic orbitals) calculations.

Dissociative Water Adsorption by AlO in the Gas Phase.

We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory (DFT) to study the adsorption of up to four water molecules on AlO. The infrared photodissociation spectra of [AlO(DO)] are measured in the O-D stretching (3000-2000 cm) as well as the fingerprint spectral region (1300-400 cm) and are assigned based on a comparison with simulated harmonic infrared spectra for global minimum-energy structures obtained with DFT. We find that dissociative water adsorption is favored in all cases.

Gas-Phase Vibrational Spectroscopy of the Aluminum Oxide Anions (Al O ) AlO.

We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory to probe how the structural variability of alumina manifests itself in the structures of the gas-phase clusters (Al O ) AlO with n=1-6. The infrared photodissociation spectra of the D -tagged complexes, measured in the fingerprint spectral range (400-1200 cm ), are rich in spectral features and start approaching the vibrational spectrum of amorphous alumina particles for n>4. Aided by a genetic algorithm, we find a trend towards the formation of irregular structures for larger n, with the exception of n=4, which exhibits a C ground-state structure.

[Al2O4](-), a Benchmark Gas-Phase Class II Mixed-Valence Radical Anion for the Evaluation of Quantum-Chemical Methods.

The radical anion [Al2O4](-) has been identified as a rare example of a small gas-phase mixed-valence system with partially localized, weakly coupled class II character in the Robin/Day classification. It exhibits a low-lying C2v minimum with one terminal oxyl radical ligand and a high-lying D2h minimum at about 70 kJ/mol relative energy with predominantly bridge-localized-hole character. Two identical C2v minima and the D2h minimum are connected by two C2v-symmetrical transition states, which are only ca.

Gas phase structures and charge localization in small aluminum oxide anions: Infrared photodissociation spectroscopy and electronic structure calculations.

We use cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations to study the structure of mono- and dialuminum oxide anions. The infrared photodissociation spectra of D2-tagged AlO1-4 (-) and Al2O3-6 (-) are measured in the region from 400 to 1200 cm(-1). Structures are assigned based on a comparison to simulated harmonic and anharmonic IR spectra derived from electronic structure calculations.

Numerically accurate linear response-properties in the configuration-interaction singles (CIS) approximation.

In the present work, we report an efficient implementation of configuration interaction singles (CIS) excitation energies and oscillator strengths using the multi-resolution analysis (MRA) framework to address the basis-set convergence of excited state computations. In MRA (ground-state) orbitals, excited states are constructed adaptively guaranteeing an overall precision. Thus not only valence but also, in particular, low-lying Rydberg states can be computed with consistent quality at the basis set limit a priori, or without special treatments, which is demonstrated using a small test set of organic molecules, basis sets, and states.

Regularizing the molecular potential in electronic structure calculations. II. Many-body methods.

In Paper I of this series [F. A. Bischoff, "Regularizing the molecular potential in electronic structure calculations.

Regularizing the molecular potential in electronic structure calculations. I. SCF methods.

We present a method to remove the singular nuclear potential in a molecule and replace it with a regularized potential that is more amenable to be represented numerically. The singular nuclear potential is canceled by the similarity-transformed kinetic energy operator giving rise to an effective nuclear potential that contains derivative operators acting on the wave function. The method is fully equivalent to the non-similarity-transformed version.

Computing molecular correlation energies with guaranteed precision.

We present an approach to compute accurate correlation energies for atoms and molecules in the framework of multiresolution analysis (MRA), using an adaptive discontinuous multiresolution spectral-element representation for the six-dimensional (two-electron) pair function. The key features of our approach that make it feasible, namely (1) low-rank tensor approximations of functions and operators and (2) analytic elimination of operator singularities via explicit correlation, were retained from the previous work [F. A.

Computing many-body wave functions with guaranteed precision: the first-order Møller-Plesset wave function for the ground state of helium atom.

We present an approach to compute accurate correlation energies for atoms and molecules using an adaptive discontinuous spectral-element multiresolution representation for the two-electron wave function. Because of the exponential storage complexity of the spectral-element representation with the number of dimensions, a brute-force computation of two-electron (six-dimensional) wave functions with high precision was not practical. To overcome the key storage bottlenecks we utilized (1) a low-rank tensor approximation (specifically, the singular value decomposition) to compress the wave function, and (2) explicitly correlated R12-type terms in the wave function to regularize the Coulomb electron-electron singularities of the Hamiltonian.

The MP2-F12 method in the Turbomole program package.

A detailed description of the explicitly correlated second-order Møller-Plesset perturbation theory (MP2-F12) method, as implemented in the TURBOMOLE program package, is presented. The TURBOMOLE implementation makes use of density fitting, which greatly reduces the prefactor for integral evaluation. Methods are available for the treatment of ground states of open- and closed-shell species, using unrestricted as well as restricted (open-shell) Hartree-Fock reference determinants.

Low-order tensor approximations for electronic wave functions: Hartree-Fock method with guaranteed precision.

Here we report a formulation of the Hartree-Fock method in an adaptive multiresolution basis set of spectral element type. A key feature of our approach is the use of low-order tensor approximations for operators and wave functions to reduce the steep rise of storage and computational costs with the number of degrees of freedom that plague finite element computations. As a proof of principle we implemented Hartree-Fock method without explicit storage of the full-dimensional wave function and with guaranteed precision (microhartree precision for up to 14 electron systems is demonstrated).

Scalar relativistic explicitly correlated R12 methods.

Combinations of explicitly correlated R12 wave functions with relativistic Douglas-Kroll-Hess (DKH) Hamiltonians are discussed. We considered several ways to incorporate the relativistic terms into the second-order Moller-Plesset R12 method and applied them to the helium isoelectronic series to investigate their accuracy and numerical stability. Among the approaches are the evaluation of the relativistic terms via double resolution-of-the-identity and the explicit evaluation of all terms up to O(c(-4)) using the Pauli Hamiltonian.

Second-order electron-correlation and self-consistent spin-orbit treatment of heavy molecules at the basis-set limit.

Second-order perturbation theory using explicitly correlated wave functions has been introduced into a quasirelativistic two-component formalism. The convergence of the correlation energy is as much improved as for the nonrelativistic Hamiltonian, achieving basis-set-limit results in a moderate-size basis set. Equilibrium distances and vibrational frequencies of small molecules of the 6th period of the periodic system of the elements have been calculated, demonstrating the improved behavior of the explicitly correlated wave functions.