Strain Dependence of Phonons in α-Uranium

The phonons in orthorhombic α-uranium have been well studied due to the charge density wave state, yet a systematic study of the phonons as a function of strain has yet to be accomplished. Here we compute the phonons and anharmonicity in α-uranium using the irreducible derivative method, guaranteeing the highest computational efficiency for a finite displacement method. Various functionals within density functional theory are used to study the change in phonons with respect to the lattice parameters, with particular focus on changes in the soft-modes. Soft phonon modes other than the mode associated with the charge density wave are also found, which have not yet been identified in previous studies. Quartic phonon interactions associated with all soft modes are computed, allowing for the computation of thermal expansion within the Hartree-Fock approximation for phonons. Our findings are compared with existing experimental data.