Delta NO and the complexities of electron correlation in simple hydrogen clusters

摘要

The Delta natural orbital (Delta NO) two-electron density matrix (2-RDM) and energy expression are derived from a multideterminantal wave function. The approximate Delta NO 2-RDM is combined with an on-top density functional and a double-counting correction to capture electron correlation. A trust-region Newton's method optimization algorithm for the simultaneous optimization of Delta NO orbitals and occupancies is introduced and compared to the previous iterative diagonalization algorithm. The combination of Delta NO and two different on-top density functionals, Colle-Salvetti (CS) and Opposite-spin exponential cusp and Fermi-hole correction (OF), is assessed on small hydrogen clusters and compared to density functional, single-reference coupled-cluster, and multireference perturbation theory (MRMP2) methods. The Delta NO-CS and Delta NO-OF methods outperform the single-reference methods and are comparable to MRMP2. However, there is a distinct qualitative error in the Delta NO potential energy surface for H-4 compared to the exact. This discrepancy is explained through analysis of the Delta NO orbitals, occupancies, and the two-electron density. Published under an exclusive license by AIP Publishing.