Charge delocalization error in Piris natural orbital functionals

Abstract

Piris Natural Orbital Functionals (PNOFs) have been recognized as a low-scaling alternative to study strong correlated systems. In this work, we address the performance of the fifth functional (PNOF5) and the seventh functional (PNOF7) to deal with another common problem, the charge delocalization error. The effects of this problem can be observed in charged systems of repeated well-separated fragments, where the energy should be the sum of the charged and neutral fragments, regardless of how the charge is distributed. In practice, an energetic overstabilization of fractional charged fragments leads to a preference for having the charge delocalized throughout the system. To establish the performance of PNOFs regarding charge delocalization error, charged chains of helium atoms and the W4-17-MR set molecules were used as base fragments, and their energy, charge distribution, and correlation regime were studied. It was found that PNOF5 prefers localized charge distributions, while PNOF7 improves the treatment of interpair static correlation and tends to the correct energetic limit for several cases, although a preference for delocalized charge distributions may arise in highly strong correlation regimes. Overall, it is concluded that PNOFs can simultaneously deal with static correlation and charge delocalization errors, resulting in a promising choice to study charge-related problems.