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We present a detailed investigation of observable properties associated with the relative stability of the rocksalt (B1) and cesium chloride (B2) phases in the AX (AϭLi, Na, K, Rb, Cs; XϭF, Cl, Br, I͒ crystal family. Thermodynamic B1→B2 transition pressures and ⌬Y ϭY (B2)ϪY (B1) differences in total energies, volumes, and bulk moduli at zero and transition pressures are computed following a localized Hartree-Fock method. The arrangement of the data in clear trends is shown to be mainly dominated by the cation atomic number. This behavior is well interpreted in terms of a variety of microscopic arguments that emerge from ͑i͒ the evaluation of the energy Hessian at the B1 and B2 points and ͑ii͒ the decomposition of the energy and pressure in anionic and cationic classical and quantum-mechanical contributions.

The structural and thermodynamic properties of MgF2 have been investigated in a wide range of pressures (0−80 GPa) and temperatures (0−850 K) by coupling quantum-mechanical ab initio perturbed ion calculations with a quasi-harmonic Debye model. The room temperature, zero-pressure structural parameters and lattice energy are computed with errors smaller than 2% when correlation energy corrections are incorporated in the calculation. Our computed equation of state is compatible with direct measurements of the bulk modulus and obeys universal p−V relations. We have simulated the rutile-to-fluorite phase transition during the loading process and have found lower (≃4 GPa) and upper (≃45 GPa) bounds for the transition pressure by means of thermodynamic and mechanical criteria for phase stability. Bonding properties and their change with pressure have been derived through a topological analysis of the electron density using Bader's theory of atoms in molecules. This analysis reveals that MgF2 is a highly ionic compound. Its ionicity decreases linearly with increasing pressure and, as in other ionic compounds, the crystal shows anion−anion bonds.

In this paper we study the chemical bonding of the small (monomer, triatomic, and dimer) neutral clusters of AlN, GaN, and InN presented earlier in paper 1. It includes the analysis of the topology of the electron density and its Laplacian, together with relevant atomic properties, in light of the theory of atoms in molecules. The most prominent feature of the bonding here, the existence of strong N-N bonds, is seen to diminish with an increase in the number of metal atoms and the degree of ionicity. The Al-N bond shows a large transfer of charge, but also a significant deformation of the Al electron shells, so it can be understood as a highly polar shared interaction. On the other hand, Ga-N and In-N bonds are nonshared interactions, with smaller charge transfers and polarizations. In all cases, the existence of a N-N bond weakens the metalnitrogen bond. The bonding picture that emerges depends only on the reliability of the electron densities, and it is consistent with the conclusions of our previous work in paper 1.