Sesquiterpene synthase that catalyzes the formation of germacrene D from trans,trans-farnesyl diphosphate (FPP). (2E,6E)-farnesyl diphosphate = (-)-germacrene D + diphosphate Binds 3 Mg(2+) or Mn(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.