Nonexcitonic mechanism for electronic and structural phase transitions in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Ta</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mi>Ni</mml:mi> <mml:msub> <mml:mrow> <mm

The authors challenge a long-standing view of Ta${}_{2}$NiSe${}_{5}$ as an excitonic insulator by revealing a nonexcitonic origin for its phase transitions. Using systematic density functional theory calculations across the Ta${}_{2}$Ni(Se,S)${}_{5}$ series, they show that structural distortions and charge doping account for the observed metal-insulator transitions. The calculations explain measured angle-resolved photoemission spectra with excellent accuracy, reconciling conflicting experimental observations and establishing a lattice-driven mechanism for the transitions in this system.