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RESEARCH PRODUCT

A first-principles DFT study of UN bulk and (001) surface: comparative LCAO and PW calculations.

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LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribu- tion, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low tem- peratures. All calculations indicate mixed metallic-covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and elec- tronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approaches. thermal conductivity, melting temperature, and metal density when compared with uranium dioxide that is commonly used nowadays. To predict nuclear fuel performance under different operating con- ditions and then a prolonged time in repository for used fuel, it is necessary to understand and predict material physicochemical properties. Of special importance are surface properties because commercial fuels are used as powders and UN, UC are effectively oxidized in air. The more so, numerous grain boundaries consider- ably affect material properties. Theoretical studies of uranium compounds are difficult due to a relativistic character of electron motion in the U atom core and strong electron-electron correlation. Moreover, UN is a rather complicated system because it is characterized by a mixed metal-covalent chemical bonding. The metallic part (U5f states near the Fermi level) is better described by a delocalized basis of the Plane Waves (PW), whereas the covalent part (U5f-N2p hybridization) by a Linear Combination of Atomic Orbital (LCAO) basis set. This is why in this article we compare results of both approaches. In Section ''Previous ab initio simulations on UN bulk,'' the comparison is made for the bulk properties (studied earlier experimentally), with a detailed analysis of relativistic pseudopo- tentials in Section ''Current DFT LCAO and PW calculations on UN bulk.'' In ''DFT LCAO and PW calculations on UN(001) surface'' section, we discuss—for the first time—the (001) sur- face properties (so far, the atomistic simulations on U compound substrates were performed only for densely packed UO2 surfaces 2 ).