With the aim of studying the characteristics of the electronic states at the Fermi energy. we habe inbestigated the electronic structure of NiH by means of an ab initio augmented plane wake calculation. The d band holes of the pure Ni metal are filled in the hydride. in agreement with magnetisation measurements and as suggested by a comparative study of the K absorption edges oi Ni and NiH. This feature is qualitatively similar to that found for PdH: however. the Fermi level in NiH is much closer to the top of the d bands than in PdH. This has se\eral important consequences. (i) NiH has a higher density of states with 10.78 electrons per Ryd per unit cell at the Fermi energy compared to 6.81 for PdH. This trend is i n agreement with the observed increase in the electronic specific heat from PdH, to NiH,. (ii) A larger metal d character and a smaller hydrogen s character is found in NiH compared to PdH, at the Fermi energy. The bandstructure results are used in conjunction with recent experimental estimates of the Debye and Einstein temperatures for the acoustic and optic modes respectively, to calculate the electron-phonon coupling parameter i using the model of Gaspari and Gyorffy. A large decrease of the electronic contribution to i is found from PdH to NiH. This contributes, together with the hardening of the optic modes evidenced by electrical resistivity measurements, to a drastic lowering of the superconducting transition temperature T, and should lead to the nonoccurrence of superconductivity in NiH above 1.5 K. A metal-hydrogen bonding band is found below the metal d states and lies much lower in energy than in the case of PdH; the bandstructure results are in general agreement with the calculation of Switendick. A site and angular momentum density-of-states analysis of the occupied energy bands is also presented which hopefully will stimulate photoemission and x-ray emission experiments.

Illinois 601 15. USC M S . received 14th M a y 1970Ahtract. Starting from bare-ion pseudopotentials, net crystal potentials are evalutiied in direct space by (a) the usual linear screening technique and (b) the Thomas-Fermi method. For the metals considered (Na and AI), the potentials are quite similar, crossing outside the cores and never differing by more than 0.02 a.u. in the case of Na nor by more than 0.2 a.u. in the case of Al. The similarity of the results based on (a) and (b) suggest the use of the pseudopotential concept in conjunction with the rather general method (b) under conditions when (a) is questionable, that is whenever the zeroth order uniform electron gas solution is a poor physical approximation to the actual system under study (specific examples being molecules, insulating solids and interfaces).Methods (a) and (bj both yield shallower wells than those most often used as input information in band structure calculations and obtained by superposing atomic data.

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