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Photomechanical molecular crystals have garnered attention for their ability to transform light into mechanical work, but difficulties in characterizing the structural changes and mechanical responses experimentally have hindered the development of practical organic crystal engines. This study proposes a new computational framework for predicting the solid-state crystal-to-crystal photochemical transformations entirely from first principles, and it establishes a photomechanical engine cycle that quantifies the anisotropic mechanical performance resulting from the transformation. The approach relies on crystal structure prediction, solid-state topochemical principles, and high-quality electronic structure methods. After validating the framework on the well-studied [4 + 4] cycloadditions in 9-methyl anthracene and 9- tert -butyl anthracene ester, the experimentally-unknown solid-state transformation of 9-carboxylic acid anthracene is predicted for the first time. The results illustrate how the mechanical work is done by relaxation of the crystal lattice to accommodate the photoproduct, rather than by the photochemistry itself. The large ∼10 7 J m −3 work densities computed for all three systems highlight the promise of photomechanical crystal engines. This study demonstrates the importance of crystal packing in determining molecular crystal engine performance and provides tools and insights to design improved materials in silico . 

( E )-4-Fluoro-cinnamaldehyde malononitrile (( E )- 4FCM ) is a new phenylbutadiene derivative that undergoes a [2+2] photocycloaddition in the crystal form. Optical absorption and proton nuclear magnetic resonance ( 1 H-NMR) measurements demonstrate that the solid-state ( E )- 4FCM photodimerization is a negative photochromic reaction that proceeds to 97% completion. The large geometry change and full conversion allow bulk crystals of ( E )- 4FCM to show strong photosalient effects when exposed to 405 nm ultraviolet light. When ( E )- 4FCM nanowires are grown in an anodic alumina oxide (AAO) template, they maintain a high degree of crystallinity and orientation, as determined by X-ray diffraction measurements. When illuminated, ( E )- 4FCM nanowire bundles exhibit a rapid expansion, during which they spread by as much as 300% in the lateral direction. This lateral expansion is at least partially due to a photoinduced crystal expansion along the diameter of the nanowires. When ( E )- 4FCM nanowires are confined inside the AAO template, the photoinduced expansion can be harnessed to deform the template, causing it to bend under UV light irradiation. The bending motion due to 2.0 mg of 4FCM in a template can cause the template to bend by up to 1.0 mm and lift up to 200 g. These results represent a significant improvement in work output relative to previous composite actuator membranes based on diarylethene photochromes.