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Heats of reaction to form co-crystals can be calculated by subtracting the heats of formation of the components (in their solid state) from the heat of formation of the co-crystal. Thus for 4,4'-Biphenol bis(caprolactam) (KEWZUI), the PM7 DHf of the co-crystal is -273.2 kcal/mol, and for 4,4'-biphenol and caprolactam, -87.7 and -82.7 kcal/mol, respectively. From these numbers, the heat of formation of the co-crystal is given by: -273.2 - (-87.7) -2x(-82.7) or -20.1 kcal/mol. All the reference structures for the co-crystals examined were taken from the Cambridge Structural Database, and by implication, the co-crystals are all more stable than the precursors. From this it follows that the DHr must be negative. This criterion is achieved for most co-crystals in PM7.