ABSTRACTŽ Second-order density functional methods where the correlation energy depends on the . second-order density matrix and on a density functional are used to introduce the Ž . electron correlation in two-configuration direct minimization TCDM ab initio electronic Ž . energy calculations of three-dimensional potential energy surfaces PES . We analyzed the behavior of these methods in PES calculations by applying them to the Be q FH reaction. This system was studied also by the usual techniques, allowing a full comparison for the lowest 1 A X adiabatic state. In particular, we compared the results obtained using the TCDM and multiple reference single and double excitations configuration interaction Ž . Ž . MRDCI methods with the corresponding results obtained using the Colle᎐Salvetti CS Ž . and Moscardo᎐San Fabian MSF procedures, within the correlation factor method, usinǵá s starting point the TCDM results. We found that the CS and MSF results are in a good overall agreement with the more accurate ab initio results, including the heights of the saddle points and the transition state.

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