𝑈 !"## is the athermal lattice energy. 𝑈 $%& and 𝑆 $%& denote the vibrational internal energy and vibrational entropy from phonon contribution under the harmonic model, respectively. Figure shows the Helmholtz free energy difference ΔA relative to the lower energy phase as a function of temperature. Between 0-1000 K, the 1T-Li 2 NiO 2 is always lower in energy and higher temperature gives greater stabilization over the I-Li 2 NiO 2 phase. Therefore, this method failed to describe the phase stability of Li 2 NiO 2 or predict any temperature-induced phase transitions with the chosen functional. Some forms of anharmonicity (e.g. associated with the square planar [NiO 4 ] geometry), may have been neglected in the calculations.