  Crystal structure prediction with theoretical methods is particularly
challenging when unit cells with many atoms need to be considered. Here we
employ a symmetry-driven structure search (SYDSS) method and combine it with
density functional theory (DFT) to predict novel crystal structures at high
pressure. We sample randomly from all 1,506 Wyckoff positions of the 230 space
groups to generate a set of initial structures. During the subsequent
structural relaxation with DFT, existing symmetries are preserved, but the
symmetries and the space group may change as atoms move to more symmetric
positions. By construction, our algorithm generates symmetric structures with
high probability without excluding any configurations. This improves the search
efficiency, especially for large cells with 20 atoms or more. We apply our
SYDSS algorithm to identify stoichiometric (H2O)_n-(NaCl)_m and C_nO_m
compounds at high pressure. We predict a novel H2O-NaCl structure with Pnma
symmetry to form at 3.4 Mbar, which is within the range of diamond anvil
experiments. In addition, we predict a novel C2O structure at 19.8 Mbar and C4O
structure at 44.0 Mbar with Pbca and C2/m symmetry respectively.
