Understanding the difference in cohesive energies between alpha and beta tin in DFT calculations

The transition temperature between the low-temperature alpha phase of tin to beta tin is close to the room temperature (Tαβ = 130C), and the difference in cohesive energy of the two phases at 0 K of about ΔEcoh =0.02 eV/atom is at the limit of the accuracy of DFT (density functional theory) with available exchange-correlation functionals. It is however critically important to model the relative phase energies correctly for any reasonable description of phenomena and technologies involving these phases, for example, the performance of tin electrodes in electrochemical batteries. Here, we show that several commonly used and converged DFT setups using the most practical and widely used PBE functional result in ΔEcoh ≈0.04 eV/atom, with different types of basis sets and with different models of core electrons (all-electron or pseudopotentials of different types), which leads to a significant overestimation of Tαβ. We show that this is due to the errors in relative positions of s and p –like bands, which, combined with different populations of these bands in α and β Sn, leads to overstabilization of alpha tin. We show that this error can be effectively corrected by applying a Hubbard +U correction to s –like states, whereby correct cohesive energies of both α and β Sn can be obtained with the same computational scheme. We quantify for the first time the effects of anharmonicity on ΔEcoh and find that it is negligible.