As expected, van der Waals (vdW) dispersion interactions are found to impact the structural parameters and relative energies of these low-dimensional pure-covalent compounds, compressing unit cell volumes by ⇠ 25 % on average (Table ). The gmonoclinic (P2 1 /c space group) polymorph is predicted to be the lowest energy phase, with t-Se (P3 1 21) being slightly higher in energy (DE = 6 meV/atom) -neglecting temperature and pressure effects. Finite-temperature effects are expected to favour t-Se (having the highest symmetry of all Se allotropes), though we find that free energy contributions from harmonic phonons (Fig. and Table ) do not result in a thermodynamic preference for t-Se over (monoclinic) g-Se at elevated temperatures. Anharmonic contributions or residual errors in the underlying DFT functional / dispersion corrections could also affect the predicted relative energies, and there could be a kinetic preference for t-Se chain growth during crystallisation -typically achieved via low-temperature annealing of amorphous Se. * The energy differences listed in Table are small, all being less than thermal energy at room temperature (k B T ⇠ 25 meV/particle). This indicates a possible difficulty in obtaining phase-pure samples of t-Se, with the potential for low-energy metastable Se chain rearrangements or ring-like formations in the bulk of the materiala point revisited in the discussion.