Crystal chemical space is vast, and only a fraction of it has been explored. To guide expensive experimental efforts, computational materials design is increasingly applied to screen crystal chemical space to discover elemental compositions and crystal structures with desired functionality. But how many of these are realisable and how can we avoid 'plausible predictions of fantasy materials' ? When a certain chemical composition in a crystal structure is predicted to exhibit a desired set of properties, the question one must ask is whether this material will be stable and synthesisable. In virtual materials screening studies, the stability is often defined in terms of athermal internal energies, typically referred to as total energies, and (synthesisable) materials with an energy above the energetic convex hull are termed metastable.