We present a first-principles study based on density functional theory (DFT) on the electronic and structural properties of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, a layered transition metal chalcogenide that has been considered as a possible candidate for an excitonic insulator. Our systematic DFT results however provide a nonexcitonic mechanism for the experimentally observed electronic and structural phase transitions in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, in particular explaining why sulfur substitution of selenium reduces the distortion angle in the low-temperature phase and potassium dosing closes the gap in the electronic structure. Moreover, the calculations show that these two effects couple to each other. Further, our first-principles calculations predict several changes in both the crystal structure and electronic structure under the effects of uniform charge dosing and uniaxial strain, which could be tested experimentally.

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• Electrical properties
• Phase transitions
• Angle-resolved photoemission spectroscopy
• Density functional theory