  Rocksalt is the most common crystal structure among binary compounds.
Moreover, no long-lived, metastable polymorphs are observed in compounds with
the rocksalt ground state. We investigate the absence of polymorphism and the
dominance of the rocksalt structure via first-principles random structure
sampling and transformation kinetics modeling of three rocksalt compounds: MgO,
TaC, and PbTe. Random structure sampling reveals that for all three systems the
rocksalt local minimum on the potential energy surface is far larger than any
other, making it statistically the most significant structure. The kinetics
modeling shows that virtually all other relevant structures, including most of
the 457 known A$_1$B$_1$ prototypes, transform rapidly to rocksalt, making it
the only option at ambient conditions. These results explain the absence of
polymorphism in binary rocksalts, answer why rocksalt is the structure of
choice for high-entropy ceramics and disordered ternary nitrides, and help
understand/predict metastable states in crystalline solids more generally.
