Weak Coulomb correlations stabilize the electride high-pressure phase of elemental calcium.

Theoretical studies using the state-of-the-art density functional theory and dynamicalmean-field theory (DFT + DMFT) method show that weak electronic correlation effects are crucial for reproducing the experimentally observed pressure-induced phase transitions of calcium from β-tin to Cmmm and then to the simple cubic structure. The formation of an electride state in calcium leads to the emergence of partially filled and localized electronic states under compression. The electride state was described using a basis containing molecular orbitals centered on the interstitial site and Ca-d states. We investigate the influence of Coulomb correlations on the structural properties of elemental Ca, noting that approaches based on the Hartree-Fock method (DFT + U or hybrid functional schemes) are poorly suited for describing correlated metals. We find that only the DFT + DMFT method reproduces the correct sequence of high-pressure phase transitions of Ca at low temperatures.