Proton ordered cubic and hexagonal periodic models of ordinary ice

A new theoretical model for hexagonal ice, Ih(d), from first principles investigations

Due to their great importance in science, technology, and the life sciences, water and ice have been extensively investigated over many years. In particular, hexagonal ice Ih has been of great interest since it is the most common form of ice, and several modifications, Ih(a), Ih(b) and Ih(c) are known, whose structural details are still under discussion. In this study, we present an alternative theoretical model, called Ih(d), for the hexagonal ice modification in space group P63/mmc (no. 194), based on first-principles calculations that have been performed using DFT-LDA, GGA-PBE, and hybrid B3LYP and PBE0 functionals.

The accurate calculation of the band gap of liquid water by means of GW corrections applied to plane-wave density functional theory molecular dynamics simulations

Knowledge about the intrinsic electronic properties of water is imperative for understanding the behaviour of aqueous solutions that are used throughout biology, chemistry, physics, and industry.

Surface energy and surface proton order of ice Ih

Ice Ih is comprised of orientationally disordered water molecules giving rise to positional disorder of the hydrogen atoms in the hydrogen bonded network of the lattice. Here we arrive at a first principles determination of the surface energy of ice Ih and suggest that the surface of ice is significantly more proton ordered than the bulk. We predict that the proton order-disorder transition, which occurs in the bulk at approximately 72 K, will not occur at the surface at any temperature below surface melting. An order parameter which defines the surface energy of ice Ih surfaces is also identified.

Faceting during the transformation of amorphous to crystalline ice

We study the thermally activated transition from amorphous to crystalline ice (D2O) on Cu(111) with high-resolution scanning tunneling microscopy. Annealing of amorphous solid water up to the desorption temperature of 149 K results subsequently in monomer decorated double bilayers with different superstructure, a faceted surface, pyramidal islands, and nanocrystallites of distinct height at different coverages. Though all structures are truncations from crystalline water ice, for none of them is the ice bilayer found to be the terminating surface.

A comparison between plane wave and Gaussian-type orbital basis sets for hydrogen bonded systems: formic acid as a test case

The formic acid molecule, its dimers, and its molecular crystal are adopted as test systems to compare results obtained with plane wave (PW) basis sets and norm-conserving pseudopotentials to all-electron Gaussian-type orbital (GTO) calculations. The CPMD and CRYSTAL06 codes, respectively, are applied with the PBE, PW91, and BLYP density functionals. Hydrogen bonding is the leading interaction in the dimers and the crystal. In the latter, dispersive and weak C-H...O interactions are also relevant. Irrespective of the adopted functional, for all considered structures PW and GTO results converge smoothly as a function of the quality of the adopted basis sets to the same values for structures, energies of interaction, and harmonic vibrational features. To achieve a high level of mutual agreement the use of GTO basis sets of at least of triple-zeta quality including one set of polarization functions and PW basis sets with a kinetic energy cutoff higher than 110 Ry is recommended. Pros and cons of both approaches for studying molecular crystals are also discussed.

Periodic ab initio study of structural and vibrational features of hexagonal hydroxyapatite Ca10(PO4)6(OH)2

Structural and vibrational features of hexagonal hydroxyapatite HA [Ca(10)(PO(4))(6)(OH)(2), space group P6(3)] are computed ab initio within a periodic approach using the CRYSTAL03 program and the B3LYP hybrid functional with a Gaussian-type basis set of polarized double zeta quality. Experimental lattice parameters and internal coordinates have been fully optimized and the final structure characterized by means of its band structure, density of states and Mulliken analysis. The full B3LYP harmonic vibrational spectrum of HA at Gamma point has also been computed and compares well with the available experimental IR and Raman data. Nevertheless, the presence of one negative frequency in the computed spectrum shows that, within the hexagonal symmetry imposed by the P6(3) group, the structure is a saddle point. This is at variance with the monoclinic structure (under P2(1)/b space group), which has been computed, with the same approach, to be a minimum of 17 kJ mol(-1) (per unit cell) more stable than the corresponding hexagonal HA structure.