Dispersion interactions within the Piris natural orbital functional theory: The helium dimer.

The authors have investigated the description of the dispersion interaction within the Piris natural orbital functional (PNOF) theory. The PNOF arises from an explicit antisymmetric approach for the two-particle cumulant in terms of two symmetric matrices, Δ and Λ. The functional forms of these matrices are obtained from the generalization of the two-particle system expressions, except for the off-diagonal elements of Δ. The mean value theorem and the partial sum rule obtained for the off-diagonal elements of Δ provide a prescription for deriving practical functionals. In particular, the previous employed approximation {J_pp/2} for the mean values {J_p^*} affords several molecular properties but it is incapable to account for dispersion effects. In this work, the authors analyze a new approach for J_p^* obtained by factorization of the matrix Δ within the bounds on its off-diagonal elements imposed by the positivity conditions of the two-particle reduced density matrix. Additional terms for the matrix elements of Λ proportional to the square root of the holes are again introduced to describe properly the occupation numbers of the lowest occupied levels. The authors have found that the cross products between weakly occupied orbitals must be removed from the functional form of Λ to obtain a correct long-range asymptotic behavior. The PNOF is used to predict the binding energy as well as the equilibrium distance of the helium dimer. The results are compared with the full configuration-interaction calculations and the corresponding experimental data. [ABSTRACT FROM AUTHOR]