Structural and energetic properties are studied by quantum mechanical methods for hydrogen peroxide and its isotopomers, with the purpose of elucidating the features of the internal modes, and in particular the torsion around the O-O bond which leads to the chirality changing isomerization. The cis and trans barriers are found in good agreement with published data and the dihedral angle dependence of the dipole moment and of the reaction path is discussed with reference to previous experimental and theoretical information. The pictures in terms of stretching, bending and torsion modes based both on the usual valence type description of the molecule and on orthogonal local vectors are compared and the greater accuracy of the latter is demonstrated. (c) 2006 Elsevier B.V. All rights reserved.