Anharmonic contributions to vibrational energies and absorption and vibrational circular dichroism spectral intensities are discussed for the example of trans-2,3-dideuteriooxirane. Higher energy and dipole derivatives are calculated ah initio at the Hartree-Fock level in Cartesian coordinates. The combination of variational and perturbational calculations of vibrational energies allows consistent treatment also for the case of Fermi resonances. A cubic energy term can account for the main part of the anharmonic contribution.