Atomic scale computer simulations based on density functional theory (DFT) are used to calculate the
formation energies and structures associated with phases in the U–N, Pu–N, U–C and Pu–C systems.
Stable phases across the compositional spaces, from the metal to the nitrogen gas or graphite end
members, are identified using convex hull analysis. Many predicted phases correspond to those known
from experimental phase diagrams (e.g. UN, U2N3; PuN; UC, U2C3; Pu2C3). However, many phases
only sit on the convex hull upon inclusion of a suitably characterised Hubbard parameter (i.e. DFT+U).
A nonstoichiometric composition of UN2−x is identified on the U–N convex hull but others, including
stoichiometric UN2, are close to the line. A stoichiometric structure for Pu3C2 with R3c symmetry is
identified, alongside which a nonstoichiometric PuC1−x phase has a similar energy.