The automerization of the tetramethyl-cyclobutadiene (TMCBD) has been modeled using a multicon figurational methodology (MCSCF) in order to analyze the influence of tunnelling in the experimental Raman spectrum. We successfully predict the appearance of additional bands in the experimental Raman spectrum of TMCBD which cannot be predicted within the harmonic approximation. Our results show good agreement with previous calculations on the automerization of the cyclobutadiene (CBD). (c) 2008 Elsevier B.V. All rights reserved.