AbstractThe geometry of the ground states of the isomers and transition state for the systems HCN ⇌ HNC and BCN ⇌ BNC have been investigated using a wide variety of basis sets, both at the self‐consistent‐field (SCF) level and including correlation at the second‐, and third‐order Møller–Plesset (MP2 andMP3) levels. The barrier to isomerization and the isomerization energy were shown to be strongly dependent on the basis set and method, particularly for