title: A natural orbital functional for multiconfigurational states.
abstract: An explicit formulation of the Piris cumulant λΔ,Π matrix is described herein, and used to reconstruct the two-particle reduced density matrix (2-RDM). Then, we have derived a natural orbital functional, the Piris Natural Orbital Functional 5, PNOF5, constrained to fulfill the D, Q, and G positivity necessary conditions of the N-representable 2-RDM. This functional yields a remarkable accurate description of systems bearing substantial (near)degeneracy of one-particle states. The theory is applied to the homolitic dissociation of selected diatomic molecules and to the rotation barrier of ethylene, both paradigmatic cases of near-degeneracy effects. It is found that the method describes correctly the dissociation limit yielding an integer number of electrons on the dissociated atoms. PNOF5 predicts a barrier of 65.6 kcal/mol for the ethylene torsion in an outstanding agreement with Complete Active Space Second-order Perturbation Theory (CASPT2). The obtained occupation numbers and pseudo one-particle energies at the ethylene transition state account for fully degenerate π orbitals. The calculated equilibrium distances, dipole moments, and binding energies of the considered molecules are presented. The values obtained are accurate comparing those obtained by the complete active space self-consistent field method and the experimental data.
authors.affiliation: Kimika Fakultatea, Euskal Herriko Unibertsitatea, and Donostia International Physics Center (DIPC). P.K. 1072, 20080 Donostia, Euskadi, Spain. mario.piris@ehu.es
authors.forename: M
authors.initials: M
authors.lastname: Piris
authors.forename: X
authors.initials: X
authors.lastname: Lopez
authors.forename: F
authors.initials: F
authors.lastname: Ruipérez
authors.forename: J M
authors.initials: JM
authors.lastname: Matxain
authors.forename: J M
authors.initials: JM
authors.lastname: Ugalde