The equilibrium phase boundary between single-bonded, threefold-coordinated polymeric forms of nitrogen, and the observed, triple-bonded diatomic phases, is predicted to occur at relatively low (50 + 15 GPa) pressure. This conclusion is based on extensive local-density-functional total-energy calculations for polymeric structures (including that of black phosphorus, and another with all gauche dihedral angles) and diatomic structures (including that of the observed high-pressure e-N2 phase). We believe the diatomic phase of nitrogen, observed up to 180 Gpa and room temperature, to be metastable at these conditions, and that such hysteresis enhances the prospects for the existence of a metastable polymeric form of nitrogen at ambient conditions. In this regard, we show that the black-phosphorus and cubic gauche polymeric forms of nitrogen would encounter signi6cant barriers along high-symmetry paths to dimerization at atmospheric pressure.

PACS 71.15.Mb, 71.27.+ a, 71.30.+ h Conventional band structure calculations in the local density approximation (LDA) [1 -3] are highly successful for many materials, but miss important aspects of the physics and energetics of strongly correlated electron systems, such as transition metal oxides and f-electron systems displaying, e.g., Mott insulating and heavy quasiparticle behavior. In this respect, the LDA + DMFT approach which merges LDA with a modern many-body approach, the dynamical mean-field theory (DMFT), has proved to be a breakthrough for the realistic modeling of correlated materials. Depending on the strength of the electronic correlation, a LDA + DMFT calculation yields the weakly correlated LDA results, a strongly correlated metal, or a Mott insulator. In this paper, the basic ideas and the set-up of the LDA + DMFT(X) approach, where X is the method used to solve the DMFT equations, are discussed. Results obtained with X = QMC (quantum Monte Carlo) and X = NCA (non-crossing approximation) are presented and compared, showing that the method X matters quantitatively. We also discuss LDA + DMFT results for two prime examples of correlated materials, i.e., V 2 O 3 and Ce which undergo a Mott -Hubbard metal -insulator and volume collapse transition, respectively.

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We report local-density-functional calculations of hybridization matrix elements and effective electron-electron interactions in La2Cu04 defining a general effective Hamiltonian that we propose as an appropriate starting point for many-body calculations in this material. The parameter values lend support to an Anderson lattice model. We find the impurity approximation to this are representative of cases in which the dominant electronic features are associated with planes formed of Cu and O. We utilize the density-functional method in two ways: One way is to produce the ordinary self-consistent local-density-functional solution for the ground state, 38 6650 1988 The American Physical Society CALCULATED EFFECTIVE HAMILTONIAN FOR La2Cu04 AND. . . 6651which provides an approximate band structure and estimates of hybridization matrix elements between neighboring 0(2p)-0(2p) and Cu(3d)-0(2p) orbitals. This is similar to previous work, ' 36 although we emphasize the strong 0(2p)-0(2p) overlap which has not generally been

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