A new value for the lattice energy of a-glycine was determined by combination of the experimentally measured heat of sublimation taken from literature and the quantum-chemically calculated energy difference Etot,gp - Etot,cry, where Etot,gp is the total energy of the most stable form of the compound in the gas phase (carboxylic acid) and Etot,cry the total energy of the molecule as it occurs in its crystalline form (betaine). At the highest levels of ab initio theory employed in this study this energy difference is -(28±2) kcal/mol, indicating that older work overestimated this difference significantly. The reason for the overestimation of this energy difference was determined by means of additional ab initio calculations. The lattice energy of -(67±2)kcal/mol obtained using the new value for Etot,gp - Etot,cry is significantly more positive than an older value of -103 kcal/mol frequently cited in the literature.
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