Density-functional methods, as implemented in the Amsterdam Density Functional program, are used to calculate the electron paramagnetic resonance ͑EPR͒ and electron nuclear double resonance ͑ENDOR͒ parameters of the S 2 Ϫ defect in a halide monovacancy in various alkali halides (M Z:M ϭNa, K, Rb and ZϭCl, Br, I͒ lattices. The calculations were performed on cluster in vacuo models for the defect and its lattice surroundings, involving up to 88 atoms in order to limit boundary effects. For all M Z lattices, the calculated g and 33 S hyperfine tensors of the S 2 Ϫ molecular ion are in very good agreement with the available EPR data, explicitly supporting the monovacancy model for the defect. In addition, computational results for the principal superhyperfine and quadrupole values and axes of the nearest shells of M ϩ and Z Ϫ ions are compared with experimental ENDOR data. The merits and shortcomings of the applied cluster in the vacuo method are critically evaluated.

Calculations using density functional theory are performed to study the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) properties of S 3 Ϫ and Se 3 Ϫ impurities in alkali halide lattices. Cluster in vacuo models are used to describe the defect and the lattice surroundings. The trivacancy defect model proposed in the literature is able to reproduce both the experimental principal values and directions of the g tensor for S 3 Ϫ and Se 3 Ϫ defects doped in alkali halides. The alternative monovacancy model gives rise to important discrepancies with experiment and can be discarded. For the KCl lattice, the hyperfine tensors of the S 3 Ϫ and Se 3 Ϫ molecular ions also agree well with the available experimental data, giving further evidence to the trivacancy model. In addition, for NaCl:S 3 Ϫ and KCl:S 3 Ϫ computational results for the 23 Na and 35 Cl superhyperfine and quadrupole tensors are compared with experimental ENDOR parameters.

The performance of a density functional theory approach in calculating the equilibrium bond length, dipole moment, and harmonic vibrational frequency in a series of group 6 (Cr, Mo, W) transition metal-containing diatomic molecules is evaluated. Using flexible basis sets comprised of Slater type functions, a wide range of exchange-correlation functionals is investigated. Comparing with known experimental values and published results from high-level theoretical calculations, the most suitable functional form is selected. The importance of relativistic effects is checked, and predictions are made for several unknown dipole moments. The best agreement with experimental parameters is obtained when using a general gradient approximation, while special and hybrid functional forms give less accurate results.

An extensive level of theory study is performed on diatomic chalcogen defects in alkali halide lattices by density functional theory methods. A variety of exchange correlation functionals and basis sets are used for the calculation of electron paramagnetic resonance (EPR) parameters of XY (X, Y = O, S, Se) molecular ions doped in MZ (M = Na, K, Rb and Z = Cl, Br, I) lattices. Various factors contribute to the EPR values, such as geometrical effects, the choice of basis set and functional form. A sensitivity analysis is made by comparing experimental and theoretical magnetic resonance data. A flow scheme is proposed for obtaining the best agreement between experimental and calculated g-values for chalcogen defects in alkali halides.