Comparative studies on the adsorption of small molecules at NaCl and MgO surfaces

A critical assessment of theoretical methods for finding reaction pathways and transition states of surface processes

The performance of a variety of techniques for locating transition states on potential energy surfaces is evaluated within the density functional theory framework. Diffusion of a water molecule across NaCl(001) and HCl bond breaking on the same surface are treated as general test cases; the former is an example of a low barrier diffusion process and the latter an example of a relatively high barrier covalent bond rupture event. The methods considered include the nudged elastic band (NEB), Dewar, Healy and Stewart (DHS), dimer, constrained optimization (CO), activation-relaxation technique (ART) and one-side growing string (OGS) as well as novel combinations of the DHS with growing string (DHS + GS) and DHS plus climbing image (CI-DHS). A key conclusion to come from this study is that the NEB method is relatively fast, especially when just a single (climbing) image is used. Indeed, using more images represents an unnecessary computational burden for our set of processes. The dimer method exhibits variable performance; being poor for the water diffusion processes, which have small activation energies, but much more efficient for the HCl bond breaking process which has a higher barrier. When only a poor initial guess of the transition state geometry is available, the CI-DHS scheme is one of the most efficient techniques considered. And as a means to quickly establish an approximate minimum energy pathway the DHS + GS scheme offers some potential.

Structural and Electronic Properties of Li2B4O7

The reliability of various quantum-chemical approaches for the calculation of bulk properties of lithium tetraborate Li(2)B(4)O(7) was examined. Lattice parameters and the electronic structure obtained with density-functional theory (DFT), with DFT-Hartree-Fock (HF) hybrid methods, and with the semiempirical method MSINDO were compared to available experimental data. We also compared the results at DFT level using different wave functions, either based on linear combinations of atom-centered orbitals (LCAO), or on plane waves, as implemented in the crystalline orbital programs CRYSTAL and VASP. The basis set dependence of calculated properties was investigated for the LCAO method. In the plane wave approach ultrasoft pseudopotentials (US PP), and projector-augmented wave (PAW) potentials were used to represent the core electrons. For all methods under consideration, the calculated Li(2)B(4)O(7) structure parameters are close to each other and agree within a few percent with measured values. A more pronounced method dependence was found for the band structure, the band gap and the cohesive energy. Closest agreement between theoretical and experimental results for the band gap was obtained with the DFT-HF hybrid methods while pure DFT methods underestimate and HF based methods overestimate the measured value. It was found that the calculated band gap strongly depends on the atomic basis set in the LCAO approach. The description of the core electrons considerably affects the cohesive energy obtained with the plane wave approach. Atomic charges based on a Mulliken analysis were compared to effective charges obtained from Raman spectroscopy. Electron density maps are used to analyze the character of B-O and Li-O interactions.