Calculating highly accurate thermochemical properties of condensed matter via wave functionbased approaches (such as e.g. Hartree-Fock or hybrid functionals) has recently attracted much interest. We here present two strategies providing accurate Hartree-Fock energies for solid LiH in a large Gaussian basis set and applying periodic boundary conditions. The total energies were obtained using two different approaches, namely a supercell evaluation of Hartree-Fock exchange using a truncated Coulomb operator and an extrapolation toward the full-range Hartree-Fock limit of a Padé fit to a series of short-range screened Hartree-Fock calculations. These two techniques agreed to significant precision. We also present the Hartree-Fock cohesive energy of LiH (converged to within sub-meV) at the experimental equilibrium volume as well as the Hartree-Fock equilibrium lattice constant and bulk modulus.

The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation+ Hubbard U ͑LDA+ U͒, pseudopotential self-interaction correction ͑pseudo-SIC͒, the hybrid functional ͑combined local exchange plus Hartree-Fock exchange͒, and the local spin density SIC ͑SIC-LSD͒ method. Each method treats correlation among the five Mn 3d orbitals ͑per spin͒, including their hybridization with three O 2p orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rocksalt MnO ͑neglecting the observed transition to the NiAs structure in the 90-100 GPa range͒. Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin ͑ 5 2 → 1 2 ͒ , not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+ U gives insulator-toinsulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition.

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In the present work we investigate the adequacy of broken-symmetry unrestricted density functional theory for constructing the potential energy curve of nickel dimer and nickel hydride, as a model for larger bare and hydrogenated nickel cluster calculations. We use three hybrid functionals: the popular B3LYP, Becke's newest optimized functional Becke98, and the simple FSLYP functional ͑50% Hartree-Fock and 50% Slater exchange and LYP gradient-corrected correlation functional͒ with two basis sets: all-electron ͑AE͒ Wachtersϩ f basis set and Stuttgart RSC effective core potential ͑ECP͒ and basis set. We find that, overall, the best agreement with experiment, comparable to that of the high-level CASPT2, is obtained with B3LYP/AE, closely followed by Becke98/AE and Becke98/ECP. FSLYP/AE and B3LYP/ECP give slightly worse agreement with experiment, and FSLYP/ECP is the only method among the ones we studied that gives an unacceptably large error, underestimating the dissociation energy of Ni 2 by 28%, and being in the largest disagreement with the experiment and the other theoretical predictions. We also find that for Ni 2 , the spin projection for the broken-symmetry unrestricted singlet states changes the ordering of the states, but the splittings are less than 10 meV. All our calculations predict a ␦␦-hole ground state for Ni 2 and ␦-hole ground state for NiH. Upon spin projection of the singlet state of Ni 2 , almost all of our calculations: Becke98 and FSLYP both AE and ECP and B3LYP/AE predict 1 (d x 2 Ϫy 2