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The formalism based on the total energy bifunctional (E[ρI,ρII]) is used to derive interaction energies for several hydrogen-bonded complexes (water dimer, HCN–HF, H2CO–H2O, and MeOH–H2O). Benchmark ab initio data taken from the literature were used as a reference in the assessment of the performance of gradient-free [local density approximation (LDA)] and gradient-dependent [generalized gradient approximation (GGA)] approximations to the exchange-correlation and nonadditive kinetic-energy components of E[ρI,ρII]. On average, LDA performs better than GGA. The average absolute error of calculated LDA interaction energies amounts to 1.0 kJ/mol. For H2CO–H2O and H2O–H2O complexes, the potential-energy curves corresponding to the stretching of the intermolecular distance are also calculated. The positions of the minima are in a good agreement (less than 0.2 Å) with the reference ab initio data. Both variational and nonvariational calculations are performed to assess the energetic effects associated with complexation-induced deformations of molecular electron densities. [ABSTRACT FROM AUTHOR]

An approach in which the total energy of interacting subsystems is expressed as a bifunctional depending explicitly on two functions: electron densities of the two molecules forming a complex (ρ[sub 1] and ρ[sub 2]) was used to determine the equilibrium geometry and the binding energy of several weak intermolecular complexes involving carbazole and such atoms or molecules as Ne, Ar, CH[sub 4], CO, and N[sub 2]. For these complexes, the experimental dissociation energies fall within the range from 0.48 to 2.06 kcal/mol. Since the effect of the intermolecular vibrations on the dissociation energy is rather small, the experimental measurements provide an excellent reference set. The obtained interaction energies are in a good agreement with experiment and are superior to the ones derived from conventional Kohn–Sham calculations. A detailed analysis of relative contribution of the terms which are expressed using approximate functionals (i.e., exchange-correlation E[sub xc][ρ[sub 1]+ρ[sub 2]] and nonadditive kinetic energy T[sub s][sup nad][ρ[sub 1],ρ[sub 2]]=T[sub s][ρ[sub 1]+ρ[sub 2]]-T[sub s][ρ[sub 1]]-T[sub s][ρ[sub 2]]) is made. The nonvariational version of the applied formalism is also discussed. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]