MRCI calculations are reported for O-4((3)A) using two distinct CASSCF reference wave functions, and the results compared with previous ab initio calculations. By combining the force field of the best ab initio CASSCF surface at the saddle point with the recommended estimate for the barrier height, a four-body analytic term is obtained which added to the popular double many-body expansion potential energy surface for the title system makes it fit well such properties. (C) 2002 Published by Elsevier Science B.V.