D₂

Symbol

 * 1) Deuterium-deuterium, a type of hydrogen (H$2$) molecule. In other words, dideuterium, or diatomic deuterium.

Usage notes
Deuterium should be called by the chemical symbol "$2$H", not "D". Therefore, the proper notation for diatomic deuterium is "$2$H$2$", not "D$2$".