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In calculating band structure, the local density approximation and density functional theory are widely popular and do reproduce a lot of the basic physics. Regrettably, without some fine tuning, the local density approximation and density functional theory do not generally get the details of the experimental band structure correct, in particular the band gap in semiconductors and insulators is generally found to be too small when compared with experiment. For experimentalists using commercial packages to calculate the electronic structure of materials, some caution is indicated, as some long-standing problems exist with the local density approximation and density functional theory.

The structural, elastic and electronic properties of chalcopyrite compound CuInSe₂ and CuGaSe₂ have been investigated using the full-potential linearized muffin-tin orbital method (FP-LMTO) within the frame of density functional theory (DFT). In this approach, the local density approximation is used for the exchange-correlation potential using Perdew–Wang parametrization. The equilibrium lattice parameters, bulk modulus, transition pressure, elastic constants and their related parameters such as Poisson's ratio, Young modulus, shear modulus and Debye temperature were calculated and compared with available experimental and theoretical data. They are in reasonable agreement. In this paper the electronic properties are treated with GGA + U approach, which brings out the important role played by the d-state of noble metal (Cu) and give the correct nature of the energy band gap. Our obtained results show that both compounds exhibit semi-conductor behaviour with direct band gap.