Nonvalence Correlation-Bound Anion States of Spherical Fullerenes

We present a one-electron model Hamiltonian for characterizing nonvalence correlation-bound anion states of fullerene molecules. These states are the finite system analogs of image potential states of metallic surfaces. The model potential accounts for both atomic and charge-flow polarization and is used to characterize the nonvalence correlation-bound anion states of the C₆₀, (C₆₀)₂, C₂₄₀, and C₆₀@C₂₄₀ fullerene systems. Although C₆₀ is found to have a single (s-type) nonvalence correlation-bound anion state, the larger fullerenes are demonstrated to have multiple nonvalence correlation-bound anion states

Theoretical Characterization of the Minimum-Energy Structure of (SF₆)₂

MP2 and symmetry-adapted perturbation theory calculations are used in conjunction with the aug-cc-pVQZ basis set to characterize the SF₆ dimer. Both theoretical methods predict the global minimum structure to be of <i>C</i>₂ symmetry, lying 0.07–0.16 kJ/mol below a <i>C</i>_2<i>h</i> saddle point structure, which, in turn, is predicted to lie energetically 0.4–0.5 kJ/mol below the lowest-energy <i>D</i>_2<i>d</i> structure. This is in contrast with IR spectroscopic studies that infer an equilibrium <i>D</i>_2<i>d</i> structure. It is proposed that the inclusion of vibrational zero-point motion gives an averaged structure of <i>D</i>_2<i>d</i> symmetry

Establishing the Ground State of the Disjoint Diradical Tetramethyleneethane with Quantum Monte Carlo

The nature of the electronic ground state of the tetramethyleneethane (TME) diradical has proven to be a challenge for both experiment and theory. Through the use of quantum Monte Carlo (QMC) methods and multireference perturbation theory, we demonstrate that the lowest singlet state of TME is energetically lower than the lowest triplet state at all values of the torsional angle between the allyl subunits. Moreover, we find that the maximum in the potential energy curve for the singlet state occurs at a torsional angle near 45°, in contrast to previous calculations that placed the planar structure of the singlet state as the highest in energy. We also show that the CASPT2 method when used with a sufficiently large reference space and a sufficiently flexible basis set gives potential energy curves very close to those from the QMC calculations. Our calculations have converged the singlet–triplet gap of TME as a function of methodology and basis set. These results provide insight into the level of theory required to properly model diradicals, in particular disjoint diradicals, and provide guidelines for future studies on more complicated diradical systems

Mechanism of Oxygen Exchange between CO₂ and TiO₂(101) Anatase

The mechanism of oxygen exchange between CO₂ and a defective anatase (101) surface was investigated by density functional theory calculations including corrections for long-range dispersion interactions and for on-site Coulomb interactions. The calculations identify a carbonate-like configuration at a surface oxygen defect site as the key intermediate species responsible for the oxygen exchange. The stability of this species, its vibrational frequencies, and the reaction barriers involved in the oxygen exchange mechanism are found to be highly dependent on the specific value of the Hubbard <i>U</i> correction used to describe the on-site Coulomb interactions within the GGA+U procedure. <i>U</i> parameter values that result in CO₂ adsorption energies and reaction barriers for oxygen exchange consistent with the results of room-temperature experiments are smaller (<i>U</i> ≤ 2.5 eV) than those that reproduce the experimental band gap or location of defect states in the band gap of the reduced TiO₂ crystal

Diffusion of CO₂ on the Rutile TiO₂(110) Surface

The diffusion of CO₂ molecules on a reduced rutile TiO₂(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO₂ molecule at an oxygen vacancy (V_O) becomes increasingly streaky with increasing temperature, indicating thermally activated CO₂ diffusion from the V_O site. From temperature-dependent tunneling current measurements, the barrier for diffusion of CO₂ from the V_O site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO₂ along Ti rows of only 1.33 kcal/mol

Water Chain Formation on TiO₂(110)

The adsorption of water on a reduced rutile TiO₂(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM measurements show that at a temperature of 50 K, an isolated water monomer adsorbs on top of a Ti­(5f) atom on the Ti row in agreement with earlier studies. As the coverage increases, water molecules start to form one-dimensional chain structures along the Ti row direction. Supporting DFT calculations show that the formation of an H-bonded one-dimensional water chain is energetically favorable compared to monomer adsorption. In the chain, there are H-bonds between adjacent water molecules, and the water molecules also form H-bonds to neighboring bridging oxygens of TiO₂(110). Thermal annealing at <i>T</i> = 190 K leads to the formation of longer chains facilitated by the diffusion of water on the surface. The results provide insight into the nature of the hydrogen bonding in the initial stage of wetting of TiO₂

Correlation Consistent Gaussian Basis Sets for H, B–Ne with Dirac–Fock AREP Pseudopotentials: Applications in Quantum Monte Carlo Calculations

In this paper, we introduce correlation consistent Gaussian-type orbital basis sets for the H and B–Ne atoms for use with the CASINO Dirac–Fock AREP pseudopotentials. These basis sets are tested in coupled cluster calculations on H₂, B₂, C₂, N₂, O₂, and F₂ as well as in quantum Monte Carlo calculations on the water monomer and dimer and the water–benzene complex, where they are found to give low variances in variational Monte Carlo calculations and to lead to reduced time step errors and improved convergence in diffusion Monte Carlo calculations compared to the use of nonoptimized basis sets. The use of basis sets with a large number of contracted <i>s</i> and <i>p</i> primitives is found to be especially important for the convergence of the energy in the diffusion Monte Carlo calculations

Dispersion-Corrected Density Functional Theory and Classical Force Field Calculations of Water Loading on a Pyrophyllite(001) Surface

Water adsorption on the (001) surface of pyrophyllite [Al­(OH)­(Si₂O₅)] was investigated using density functional theory (DFT) with dispersion corrections and force field calculations. The DFT calculations show that a water molecule can bind either to one or to two basal oxygen atoms of the surface, with adsorption energies varying from −0.10 to −0.19 eV depending on the binding configuration and binding site. Because the water–water interactions are stronger than the water–surface interactions, the energetically preferred structures with two or more molecules on the surface are clusters reminiscent of their gas-phase counterparts. The trend in water–surface binding energies with the number of water molecules obtained from force field calculations qualitatively agrees with that predicted by the dispersion-corrected DFT calculations. However, the force field calculations give a low-energy structural motif with a water molecule coordinated to a hydroxyl group associated with the octahedral layer of the pyrophyllite surface. This binding motif is found to be unstable in the DFT calculations

Molecular Dynamics Simulations of Turbostratic Dry and Hydrated Montmorillonite with Intercalated Carbon Dioxide

Molecular dynamics simulations using classical force fields were carried out to study energetic and structural properties of rotationally disordered clay mineral–water–CO₂ systems at pressure and temperature relevant to geological carbon storage. The simulations show that turbostratic stacking of hydrated Na- and Ca-montmorillonite and hydrated montmorillonite with intercalated carbon dioxide is an energetically demanding process accompanied by an increase in the interlayer spacing. On the other hand, rotational disordering of dry or nearly dry smectite systems can be energetically favorable. The distributions of interlayer species are calculated as a function of the rotational angle between adjacent clay layers

Assessing the Performances of Dispersion-Corrected Density Functional Methods for Predicting the Crystallographic Properties of High Nitrogen Energetic Salts

Several density functional methods with corrections for long-range dispersion interactions are evaluated for their capabilities to describe the crystallographic lattice properties of a set of 26 high nitrogen-content salts relevant for energetic materials applications. Computations were done using methods that ranged from adding atom–atom dispersion corrections with environment-independent and environment-dependent coefficients, to methods that incorporate dispersion effects via dispersion-corrected atom-centered potentials (DCACP), to methods that include nonlocal corrections. Among the functionals tested, the most successful is the nonlocal optPBE-vdW functional of Klimeš and Michaelides that predicts unit cell volumes for all crystals of the reference set within the target error range of ±3% and gives individual lattice parameters with a mean average percent error of less than 0.81%. The DCACP, Grimme’s D3, and Becke and Johnson’s exchange-hole (XDM) methods, when used with the BLYP, PBE, and B86b functionals, respectively, are also quite successful at predicting the lattice parameters of the test set