The application of the Restricted Active Space Self Consistent Field (RASSCF) quantum chemical method using an extended active space and including sigma-sigma, pi-sigma and pi-pi dynamical electron correlation shows that the transition state structure for the automerization reaction of cyclobutadiene is an isosceles trapezium. This transition state is obtained without any symmetry constraints. The calculated energy barrier height involving the zero point vibrational energy corrections is 9.62 kcal.mol(-1) (0.417 eV), with the corresponding rate constant being equal to 0.18 x 10(9)s(-1) (or 7.1 x 10(10)s(-1) in case of using the vibrational energy splitting tunneling method).