We present a modification of the optimal control theory algorithm, which enables us to control the absolute phase of a wavepacket and thereby provides the possibility to control momentum. localization and shape of a wavepacket simultaneously. This algorithm is used to manipulate the reaction velocity of the electrocyclic ringopening of cyclohexadiene, an ultrafast reaction mediated by several accessible conical intersections. Our quantum dynamical studies showed that especially one conical intersection is responsible for the fast relaxation process. It is possible to inhibit this favored pathway and thereby reduce the reaction velocity by using an optimized ultrashort laser pulse. (c) 2005 Elsevier B.V. All rights reserved.