The physical origins of the C-O rotations in glycine are explored theoretically. By the localized molecular orbital energy decomposition analysis (LMO-EDA) method, the rotation barriers are decomposed into the electrostatic, exchange-repulsion, polarization, correlation and geometrical relaxation terms. In general, the C-O rotations are controlled by Pauli repulsion and polarization interactions. However, if the rotated conformer has obvious inter-group interaction between COOH and NH2, the physical origin of the C-O rotation is changed, which is governed by polarization and correlation interactions. (C) 2015 Elsevier B.V. All rights reserved.