TRANSITION-STATES FOR HYDROGEN RADICAL REACTIONS - LIFH AS A STRINGENT TEST-CASE FOR DENSITY-FUNCTIONAL METHODS

The Becke three-parameter Lee-Yang-Parr (B3LYP) density functional met hod is applied to the study of the reaction Li + HF --> H + LiF; the r esults obtained are compared with experiment and previous multireferen ce singles and doubles configuration interaction (MRDCI) calculations, and with singles and doubles quadratic configuration interaction (QCI SD) and Gaussian 2 (G2) model chemistry calculations performed also in this paper. It is found that, using an extended 6-311++G(3df, 3pd) ba sis set, the predicted stabilization enthalpy of the initial LiFH comp lex (-34.6 kJ mol(-1)) and the exoergicity of the reaction (-7.3 kJ mo l(-1)) are predicted in agreement with experiment (similar to -31 kJ m ol(-1) and -4.8 +/- 8 kJ mol(-1), respectively). These results are als o in agreement with the MRDCI calculations (similar to -31.5 kJ mol(-1 ) and -9 kJ mol(-1), respectively). However, the energy of the transit ion state with respect to the reactants at the B3LYP level is about 25 kJ mol(-1) lower than the MRDCI result (which agrees with the QCISD a nd G2 values). Therefore, it is concluded that B3LYP is not describing this reaction properly. It is shown, however, that this defect is due mainly to the inclusion of the LYP correlation functional instead of the Perdew-Wang originally considered by Becke for developing his adia batically connected functional. When B3PW91 is considered instead of B 3LYP, the height of the transition state is in better agreement with t he conventional ab initio methods, although still off by 50%.