Sesquiterpene synthase involved in germacrene D biosynthesis. Produces also at least 13 additional sesquiterpene products, including germacrene C and (+)-germacrene A, beta-ylangene, (E)-beta-farnesene and (E,E)-alpha-farnesene. (2E,6E)-farnesyl diphosphate = (-)-germacrene D + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.