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A Lifson-Warshel force field (1968), which goes beyond the harmonic approximation and is well adapted for calculations in defect structures, has been developed for the silicon crystal. The parameters of this field have been determined by least-squares fitting of the phonon dispersion curves, which are reproduced with an accuracy comparable with that of harmonic valence force fields. Several of these parameters were kept constant during this fitting at values derived from silicon molecular data. The stacking fault energy, which vanishes for harmonic valence force fields, is calculated in reasonable agreement with experiment.

SCF-Xα-SW calculations have been performed on a Si 16 H 22 disordered cluster built according to the CRN theory. The computed electronic structure has been analysed and compared with several experimental optical and photo-electronic results obtained for a-Si and a-SiH.

The core structure of the reconstructed 90° partial dislocation in silicon is obtained by energy minimization with respect to the Lifson—Warshel valence force field using periodic boundary conditions. A low strain energy attests to the stability of the reconstructed geometry. The electronic structure is calculated using a local pseudo-potential approach in an improved peripheral orbital treatment. The silicon indirect gap is found to be completely clear of dislocation levels, in agreement with experiment. Stacking-fault levels are found in the gap near the valence band edge in agreement with specialized stacking-fault energy level calculations.

A valence force field of the Lifson-Warshel type is applied to a calculation of the core structure of the 90° partial dislocation in diamond. Both unreconstructed and reconstructed bonding topologies are considered and the energy difference between the two structures is discussed.

The energetics of bond reconstruction at the core of a 90° partial dislocation in diamond has been studied by means of HFR-MO-LCAO-SCF computations on C5H10 and C10H18 model molecular clusters. The results obtained agree with previous VFF investigations and they confirm that the reconstructed geometry is energetically favoured with respect to the unreconstructed one.

The structure and thermodynamics of some gas phase molecular reactants [As 2 , As 4 , Ga(CH 3 ) 3 ] used in MBE (molecular beam epitaxy) and MO-MBE (metal-organic molecular beam epitaxy) has been studied by Hartree-Fock-Roothaan molecular-orbitals linear-combination-of-atomic-orbitals self-consistent-field (HFR-MO-LCAO-SCF) ab initio methods. A first attempt to simulate the sticking of a gallium atom at the (100) surface of GaAs has also been made.

A continuous random network model for amorphous silicon has been generated by computer. A relaxation process for this model has been simulated by using two different force fields, one of which, the Keating, is well-known, whereas the other, the Lifson-Warshel has only recently been used in this type of solid. Comparison of the final results obtained by the use of these two fields shows that it is only with the Lifson-Warshel field that good agreement is obtained for the computed average microscopic properties of the generated clusters. The success of this field is due to its inclusion of four-body and long-range interactions, which are absent in the Keating potential.

The interconversion pathways of four planar conformers of the S-methyl ester of dithiocarbaaic acid (DTCE) have been studied by means of the HF-LCAO-MO-SCF method with a minimal basis set.

Large crystals of CoxZn1−xIn2S4 (0.00 ⩽ x ⩽ 0.46) solid solution were grown from the vapour phase by chemical transport modified by the time variation of the temperature profile procedure. Co2+ ions were found, by XPS analysis, to occupy tetrahedral as well as octahedral sites of the ZnIn2S4-type structure. The magnetic properties of Co0.46Zn0.54In2S4, crystallizing in space group R 3m, were explained in terms of the short-range magnetic order arising from the presence of isolated clusters of magnetic ions.

Quantum mechanical computations on the potential energy surface of the system CO + O at an SCF–MOab initio level are presented and discussed. The calculations are performed on a minimal basis set of atomic functions. Comparison with results obtained with extended basis sets are also presented.

A continuous random network model of amorphous germanium has been generated by computer. A relaxation process for this model has been performed by using a Lifson-Warshel force field parametrized by reproducing the phonon dispersion curves of crystalline germanium. The diffraction and radial distribution functions, computed on using the generated model, are in good agreement with those obtained experimentally by means of both neutron and X-ray diffraction, although the present model is entirely independent of them. The relative density of the model is 1.005 in good agreement with some experimental results.

We have recently produced a Lifson–Warshel force field for the silicon crystal. This field, that goes further than the harmonic approximation, is particularly adapted for the calculation of defect structures. It requires five free parameters, but one of them in the field already produced, the linear bending term, is significant for the perfect lattice structure only and should be eliminated in dealing with defect structures. Also, one of the fixed parameters can be adjusted to fit precisely the experimental value of the stacking fault energy. We show that an excellent fitting of the phonon dispersion curves, in the least squares sense, is obtained with the parameters thus changed. We illustrate the use of this field in discussing the problem of reconstruction of the 90° partial dislocation in silicon.

A Lifson-Warshel force field of the type recently used by the authors to study defect structures in silicon has now been developed for diamond. The parameters of this field are determined mostly by least-squares fitting of the phonon dispersion curves, but some experimental values must also be used. The crucial term is the torsional parameter, for which values differing by a factor of two or so have been proposed, leading to stacking-fault energies ranging from 992 to 340 mJ m−2. The latter value derives from a better-based value of the torsional parameter and agrees well with a recent experimental value of 279 ± 41 mJ m−2. The possibility of lattice relaxation in the stacking fault is also discussed.

MgIn2S4 crystallizes in the Fd3m (O7h) space group with a = 10.7108(8), A. The cation distribution (x) and the deformation parameter (u) have been determined by means of three methods: Bertaut, Furuhashi, and Baltzer. A critical discussion about the coupled use of the three methods is presented.

Madelung energy calculations are performed for two different lattices in which some of AB2X4 compounds (A = M+2, B = M−3, X = O−2, S−2, Se−2, Te−2) crystalline. A gross evaluation of the covalent bond energy contribution in the same crystals is made and some considerations are draw from the comparison between the two above quoted energy contributions.

The diffraction function for amorphous silicon, computed for a Continuous Random Network is calculated and compared with experiment.

Magnetization measurements and EPR powder spectra have been performed on polycrystalline solid solutions Co x Zn 1- x Rh 2 O 4 having the spinel structure in the temperature range 4.2–300 K. Evidence for an antiferromagnetic order is obtained for the samples with 0.40 ⩽ x ⩽ 1.00. The magnetic behavior of more diluted samples is interpreted in terms of contributions from antiferromagnetic clusters of Co 2+ ions and paramagnetic ions.