Physica B 358 (2005) 158–165 Full-potential calculations of the electronic and optical properties for 1T and 2H phases of TaS 2 and TaSe 2 Ali Hussain Reshak à , S. Auluck Physics Department, Indian Institute of Technology, Roorkee, Uttaranchal 247667, India Received 20 July 2004; received in revised form 12 November 2004; accepted 4 January 2005 Abstract Thebandstructure,densityofstatesandanisotropicfrequency-dependentopticalpropertieshavebeencalculatedfor the1Tand2HphasesofTaS 2 andTaSe 2 usingthefull-potentiallinearaugmentedplanewave(FPLAPW)method.In the 1T and 2H phases, when S is replaced by Se, the unoccupied Ta-5d and chalcogen-p bands move closer to the Fermi energy E F andthebandwidthofthechalcogen-sgroupdecreases.Comparedtothe1Tphase,inthe2Hphase the occupied/unoccupied bands move towards higher/lower energies with respect to E F .Inthe1Tphase,whenSis replaced by Se, the peak positions in the imaginary part of the frequency-dependent dielectric function  2 ðoÞ move towards lower energies by 0.5eV. The single peak at 6eV in  2 ðoÞ ofthe1Tphaseissplitintotwopeaksinthe2H phase. We make a detailed comparison of the frequency-dependent reflectivity and absorption coefficient with the available experimental data. The linear muffin tin orbital method within the atomic sphere approximation (LMTO- ASA)showspooragreementwiththeexperimentaldatawhileourFPLAPWresultsgiveexcellentagreementwiththe experimental data suggesting that a better representation of the potential is essential for calculating optical properties accurately. r 2005 Elsevier B.V. All rights reserved. PACS: 70 Keywords: Electronic structure; Optical properties; Transition metal dichalcogenides; FPLAPW 1. Introduction The group VB transition metal dichalcogenides (TMDCs) TaS 2 and TaSe 2 are members of the large family of TMDCs, whose optical and electrical properties have been reviewed by Wilson and Yoffe [1]. These compounds are interesting because they exhibit a rich variety of phase transitions produced by commensurate and incommensurate charge density waves [2]. The optical and photoelectron spectra of these compounds have been extensively studied ARTICLE IN PRESS www.elsevier.com/locate/physb 0921-4526/$-see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.051 à Corresponding author. Tel.: +0110091133272344; fax: +01191133273560. E-mail address: maalidph@yahoo.co.uk (A.H. Reshak).