Diterpene cyclases that can form multiple diterpene products. (2E,6E,10E)-geranylgeranyl diphosphate + H2O = (R)-nephthenol + diphosphate (2E,6E,10E)-geranylgeranyl diphosphate = (R)-cembrene A + diphosphate (2E,6E,10E)-geranylgeranyl diphosphate + H2O = (1S,4E,8E,12E)-2,2,5,9,13-pentamethylcyclopentadeca-4,8,12-trien-1-ol + diphosphate kcat is 1.3 min(-1). Homodimer. Belongs to the terpene synthase family.