We predict optimised lattice constants of a = 11.38 and 11.77 Å for π SnS and SnSe, respectively, which are within 2% of the measured a = 11.6 and 11.97 Å. Previous studies have shown that both π phases are dynamically stable (i.e. there are no imaginary modes in the phonon dispersion curves) and energetically metastable with respect to the orthorhombic phases of SnS and the orthorhombic and rocksalt phases of SnSe. Our calculated phonon spectra confirm the dynamical stability, with real frequencies across the Brillouin zone (Figs 2 and S2 † ).