R. CAR et al.: Electronic Band Structure of SnSe 471 phys. stat. sol. (b) 86, 471 (1978) Subject classification: 13.1 ; 22 htitWt0 d i Fi8ic.a del Politecnico (a) and Gmppo Nazionale d i 8tmttura dells Materia del C.N.R. (a), Milano Electronic Band Structure of SnSe BY R. CAR (a), G. CIUCCI (b), and L. QUARTAPELLE (a)The electronic bands of SnSe are calculated along all high symmetry lines of the Brillouin zone using the local empirical pseudopotential method. Spectralization of the involved large matrices is performed by means of a new approximate folding-down method which locates the s-states of valence bands much better than the Lowdin-Brust perturbative technique. The parameters of the analytical form used to interpolate the pseudopotentials are roughly adjusted to obtain a band structure consistent with optical and photoelectric data. Satisfactory results for the general features are obtained, a better agreement requiring a more cautious tuning of pseudopotential values in the region of small wave vectors.Die elektronischen SnSe-Biinder werden liings der Hochsymmetrie-Linien der Brillouin-Zone nach dem ortlichen empirischen Pseudopotentialverfahren berechnet. Die Eigenwerte der betreffenden groBen Matrizen werden mit einem neuen Niiherungsverfahren berechnet, wodurch die Energiewerte der s-Zustinde der Biinder vie1 besser als mit der Lowdin-Brust-Technik ermittelt werden. Die Parameter der Pseudopotentiale werden SO gewiihlt, daB eine zu den optischen und photoelektrischen Angaben passende Bandstruktur entsteht. Es werden befriedigende Ergebnisse in dem wesentlichen Verlauf erreicht, wobei fur eine bessere Ubereinstimmung eine vorsichtigere Einstellung der Pseudopotentialwerte im Gebiet der kleinen Wellenvektoren erforderlich ist.

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The construction of the irreducible representations of single and double nonsymmorphic space groups is discussed. The proof is given that for any symmetry element where the nonsymmorphism plays a role there is a finite group of lowest order such that its irreducible representations engender all the allowable representations of the little group. For most high symmetry elements the order of this optimal factor group is only twice the order of the corresponding point group of the wave vector. The computational advantages of using this group instead of other known factor groups are stressed.

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