DENSITY-FUNCTIONAL THEORY WITH AN APPROXIMATE KINETIC-ENERGY FUNCTIONAL APPLIED TO STUDY STRUCTURE AND STABILITY OF WEAK VAN-DER-WAALS COMPLEXES

In view of further application to the study of molecular and atomic st icking on dust particles, we investigated the capability of the ''free ze-and-thaw'' cycle of the Kohn-Sham equations with constrained electr on density (KSCED) to describe potential energy surfaces of weak van d er Waals complexes. We report the results obtained for C6H6 ... X (X=O -2, N-2, and CO) as test cases. In the KSCED formalism, the exchange-c orrelation functional is defined as in the Kohn-Sham approach whereas the kinetic energy of the molecular complex is expressed differently, using both the analytic expressions for the kinetic energy of individu al fragments and the explicit functional of electron density to approx imate nonadditive contributions. As the analytical form of the kinetic energy functional is not known, the approach relies on approximations . Therefore, the applied implementation of KSCED requires the use of a n approximate kinetic energy functional in addition to the approximate exchange-correlation functional in calculations following the Kohn-Sh am formalism. Several approximate kinetic energy functionals derived u sing a general form by Lee, Lee, and Parr [Lee et al., Phys. Rev. A. 4 4. 768 (1991)] were considered. The functionals of this type are relat ed to the approximate exchange energy functionals and it is possible t o derive a kinetic energy functional from an exchange energy functiona l without the use of any additional parameters. The KSCED interaction energies obtained using the PW91 [Perdew and Wang, in Electronic Struc ture of Solids '91, edited by P, Ziesche and H. Eschrig (Academie Verl ag, Berlin, 1991), p. 11] exchange-correlation functional and the kine tic energy functional derived from the PW91 exchange functional agree very well with the available experimental results. Other considered fu nctionals lead to worse results. Compared to the supermolecule Kohn-Sh am interaction energies, the ones derived from the KSCED calculations depend less on the choice of the approximate functionals used. The pre sented KSCED results together with the previous Kohn-Sham ones [Wesolo wski et al, J. Phys. Chem. A 101, 7818 (1997)] support the use of the PW91 functional for studies of weakly bound systems of our interest. ( C) 1998 American Institute of Physics. [S0021-9606(98)01215-X].