This paper reports the results of a study of the robustness of the field required to generate the isomerization reaction HCN→CNH as a function of number of degrees of freedom and the complexity of the description of the dynamics. The particular reduced state representation of the control process that is tested in this paper is the reaction path method proposed by Zhao and Rice. We show that if the description of the system dynamics includes vibrational motions perpendicular to the one-dimensional reaction path and both the interactions between those vibrations and between them and the reaction path, the fields found by the conventional computational scheme represent local optima, and none of these correspond to generating a transfer of 100% of the population from the ground vibrational state of HCN to a mixture of vibrational states of CNH. Moreover, it is very difficult to find fields that will efficiently transfer population from the ground vibrational state of HCN to particular vibrational states of CNH. Comparing the optimized control fields reported in this paper with those previously obtained using simplified versions of the reaction path reduction, one finds that the complexity ͑measured by the power spectra͒ of optimal control fields increases as the dynamical description includes more degrees of freedom and then the interactions between all of the degrees of freedom. The optimal control field generated using a simpler dynamical description is not a good guide to the optimal control field associated with a more complex dynamical description. We conclude that the reaction path method of reduction of the complexity of calculation of the optimal field required to drive a particular reaction is not likely to be useful for the design of fields with which to actively control reactions of polyatomic molecules.

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