Organic crystalline solids are useful in many applications, such as organic semiconductors, in pharmaceutical formulations, and for molecular separations using porous crystals. To achieve the optimum functionality for a given application, we must be able to produce crystal structures with specific packings of the molecular building blocks. This is because different polymorphs are known to have different bulk material properties. For example, porous molecular organic crystals are known to have polymorph-dependent gas adsorption properties, and this has been used to modulate gas selectivity. Control over the crystallization of organic molecules is a long-standing problem, and the field of 'crystal engineering' predates developments in bonded frameworks such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs). Indeed, the huge success of MOFs and COFs can be explained by the structural predictability that ensues when the less directional and weaker non-covalent interactions in molecular crystals are replaced by stronger, more directional metal-organic bonding and covalent bonding in framework materials. There are reasons, however, to consider non-bonded crystalline molecular frameworks for certain applications: for example, it has proved challenging to produce large, high-quality single crystals for MOFs and COFs, whereas high-quality organic crystals are often more accessible. Further, molecular crystals can change their structure in response to guest inclusion, and this has led to applications in porosity "switching" and in the separation of organic hydrocarbons. It remains difficult, however, to predict how a candidate molecule will crystallize and, hence, to design new function: in this regard, molecular crystals are at a disadvantage with respect to MOFs and COFs. To tackle this, we have used crystal structure prediction (CSP) to guide the discovery of molecular crystals with porous host structures. In a recent study, calculated energy-structurefunction maps were used to pre-screen the probable function of molecules in silico prior to experiment; in this case, for porosity, methane storage capacity, and guest molecule selectivity. This led to the discovery of hydrogen bonded organic frameworks (HOFs) based on rigid triptycene benzimidazalones with unprecedentedly low bulk densities (0.41 g cm ) and apparent Brunauer-Emmett-Teller surface area (SA BET ) as high as 3230 m 2 g -1 . The experimental porous crystal structures were found to correspond to low density, stable regions, or 'spikes' on the energy-density representation of the predicted crystal energy landscape. We proposed that such features might be used to predict the formation of other porous HOFs in the future.