We present the first theoretical study on a pair of diterpene enantiomers of formula C(21)H(34)O(5), which were newly isolated from plants and identified as 3 beta,5 beta-dihydroxy-16 alpha/beta-methoxyhalima-13(14)-en-15,16-olide. The equilibrium geometries and harmonic vibrational frequencies of their low-lying conformers were obtained by using the AM1 and B3LYP/6-31+G(d) methods. At the optimized geometries, rotatory strengths of six excited states of each conformer were computed by the time-dependent density functional theory. The electronic circular dichroism spectra were simulated by taking Boltzmann averaging and considering the solvent effect, from which the absolute configurations of the enantiomers were determined. Their vibrational circular dichroism spectra were also predicted. (C) 2011 Elsevier B.V. All rights reserved.