Using density functional calculations in the generalized gradient approximation, the excitation energy for the formation of a threefold coordinated oxygen atom is evaluated. The bistable E-1' defect of alpha-quartz in the neutral charge state is used as a model system. The puckered configuration which shows a threefold coordinated oxygen center together with a doubly occupied silicon dangling bond is found to be at 2.7 eV higher energy than the Si-Si dimer configuration. (C) 2000 Elsevier Science B.V. All rights reserved.