In many cases,the dynamic correlation can be calculated quite accurately and at a fairly low computational cost in Kohn-Sham density-functional theory(KS-DFT),using current standard approximate functionals.However,in general,KS-DFT does not treat static correlation effects(near degeneracy)adequately which,on the other hand,can be described in wave-function theory(WFT),for example,with a multiconfigurational self-consistent field(MCSCF)model.It is therefore of high interest to develop a hybrid model which combines the best of both WFT and DFT approaches.The merge of WFT and DFT can be achieved by splitting the two-electron interaction into long-range and short-range parts.The long-range part is then treated by WFT and the short-range part by DFT.In this work the authors consider the so-called"erf long-range interaction erf(mu r_(12)/r_(12),which is based on the standard error function,and where /u,is a free parameter which controls the range of the long-/short-range decomposition.In order to formulate a general method,they propose a recipe for the definition of an optimal mu_(opt)parameter,which is independent of the approximate short-range functional and the approximate wave function,and they discuss its universality.Calculations on a test set consisting of He,Be,Ne,Mg,H2,N2,and H2O yield mu_(opt)approx=0.4 a.u..A similar analysis on other types of test systems such as actinide compounds is currently in progress;Using the value of 0.4 a.u.for mu,encouraging results are obtained with the hybrid MCSCF-DFT method for the dissociation energies of H2,N2,and H2O,with both short-range local-density approximation and PBE-type functionals.
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