Towards extending the applicability of density functional theory to weaklybound systems

While the attempts currently in progress in several groups for the rigorous inclusion of dispersion interactions in density functional theory (DFT) ca lculations mature and evolve into practical methodology, we contribute to t he debate on the applicability of current functionals to the calculation of weak interaction with a systematic investigation of a few, typical, weakly bound systems. We have used both pure DFT and a hybrid approach in which t he total interaction energy is partitioned into two parts: (a) the dispersi on energy which, in a first approximation is the contribution due to interm onomer correlations and (b) all other interactions. The first component is accurately obtained at all distances of interest by means of a well-known d amped multipolar expansion of the dispersion energy while for the second co mponent different approximations will be evaluated. The need to avoid doubl e counting a fraction of the correlation energy when using the hybrid appro ach and the choice of the appropriate functional are also discussed. We con sider four systems of increasing binding strength, namely the Ar-2 and Kr-2 dimers, the benzene dimer, the water dimer, and a few metal carbonyls. For pure DFT calculations we confirm the conclusion reached by others concerni ng (a) the strong dependence of the results on the choice of the GGA functi onal for dispersion-dominated interaction (noble gases and benzene) with th e overall tendency to yield underbinding and (b) the relatively accurate, f unctional-independent, description for that DFT gives of water, which we at tribute to the fact that this system is dominated by electrostatic interact ions. For the carbonyls we find that DFT yields results which area again st rongly dependent on the choice of the functional and show a tendency to giv e overbinding. Our hybrid method shows instead shortcomings only for the no ble gases. The problem in this case is traceable to the well-known difficul ties that all current functionals experience at medium-large intermonomer s eparations. The quality of the hybrid results improves markedly for benzene due to the large value of both dispersion and repulsive interactions at th e equilibrium distance for this dimer, which makes the balance between the two, less delicate. Excellent results are also obtained for water (for the same reason as indicated above) and more significantly for the carbonyls wh ere we find that dispersion contributes to the binding more than it could b e guessed a priori. We do not claim to have found a general solution to thi s difficult problem, but we aim at providing a quantitative assessment to w here the problems are pointing at directions from which a general solution may, eventually, emerge. (C) 2001 American Institute of Physics.