The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands (see Figure at right) for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation and most commonly occurs in trigonal bipyramidal molecules such as PF, though it can also occur in molecules with a square pyramidal geometry.  The Berry mechanism is named after R. Stephen Berry, who first described this mechanism in 1960.