Perspective on natural orbital functional theory

• Published in: International journal of quantum chemistry Vol. 114; no. 18; pp. 1169 - 1175
• Main Authors: Piris, Mario, Ugalde, Jesus M.
• Format: Journal Article
• Language: English; French; German
• Published: Hoboken Blackwell Publishing Ltd 15.09.2014; Wiley Subscription Services, Inc
• Subjects: Chemistry; interpair electron correlation; intrapair electron correlation; N-representability; natural orbital functional; Physical chemistry; Quantum physics; reduced density matrices
• ISSN: 0020-7608; 1097-461X
• DOI: 10.1002/qua.24663

The natural orbital functional (NOF) theory is briefly reviewed. The meaning of the top‐down and bottom‐up approaches for the construction of a NOF is analyzed. A particular reconstruction of the two‐particle reduced density matrix (2‐RDM) based on the cumulant expansion is discussed. The cumulant is expressed by two auxiliary matrices, which are constrained to certain bounds due to the N‐representabilty conditions of the 2‐RDM. Appropriate forms of these matrices lead to different implementations known in the literature as PNOFi (i = 1–5). The strengths and weaknesses of PNOF5 are assessed. Its main strength is its ability to deal with the intrapair electron correlation at a reasonable computational cost. Its main limitation is the absence of the interpair electron correlation. The inclusion of the missing correlation via a multiconfigurational perturbation theory is shortly described. The growing interest in methods based on NOF theory points to a promising future in this field. © 2014 Wiley Periodicals, Inc. The main strength of Piris Natural Orbital Functional 5 (PNOF5) is its ability to deal with the intrapair electron correlation at a reasonable computational cost. However, the absence of the interpair electron correlation constitutes an important limitation of the method. This missing correlation can be included via a multiconfigurational perturbation theory.