gamma D-Crystallin (H gamma DC) is a key structural protein in the human lens, whose aggregation has been associated with the development of cataracts. Single-point mutations and post-translational modifications destabilize H gamma DC interactions, forming partially folded intermediates, where hydrophobic residues are exposed and thus triggering its aggregation. In this work, we used alchemical free-energy calculations to predict changes in thermodynamic stability (Delta Delta G) of 10 alanine-scanning variants and 12 H gamma DC mutations associated with the development of congenital cataract. Our results show that W42R is the most destabilizing mutation in H gamma DC. This has been corroborated through experimental determination of Delta Delta G employing differential scanning calorimetry. Calculations of hydration free energies from the H gamma DC WT and the W42R mutant suggested that the mutant has a higher aggregation propensity. Our combined theoretical and experimental results contribute to understand H gamma DC destabilization and aggregation mechanisms in age-onset cataracts.