DFT Investigation of (Ca(OH) 2 ) n Clusters: Structural Motifs and Implications for Early-Stage Aggregation

Abstract

Understanding the early stages of calcium hydroxide formation is important for applications in cement chemistry, catalysis, and thermochemical energy storage. In this work, we present a density functional theory (DFT) study of the structures and stability of (Ca(OH) 2 ) n clusters (n = 1-15). The coordination behavior of hydroxide ligands around Ca 2+ centers reveals a systematic evolution in which terminal η 1 -OH groups are progressively replaced by higher-order µ i (i = 2-5) bridging motifs. These motifs act as fundamental structural units that drive the transformation from planar to threedimensional cluster geometries and promote increasing structural compactness. Small clusters (n ≤ 2) adopt planar µ 2 -bridged motifs, while three-dimensional structures emerge at n = 3 with the appearance of µ 3 coordination. A structural transition occurs at n = 5-6, where µ 4 motifs enable core formation. At larger sizes (n ≥ 7), µ 5 coordination and cooperative hydrogen bonding facilitate inward hydroxide migration and further core densification. Several cluster sizes (n = 6, 7, 9, 10, 11, and 13) exhibit enhanced stability and can be identified as magic-number clusters. These results provide atomistic insight into intrinsic structural tendencies and possible pre-nucleation motifs in calcium hydroxide aggregation.