Computational Quantum Chemistry for Multiple-Site Heisenberg Spin Couplings Made Simple: Still Only One Spin–Flip Required

• Published in: The journal of physical chemistry letters 2015-05, Vol.6 (10), p.1982-1988
• Main Authors: Mayhall, Nicholas J, Head-Gordon, Martin
• Format: Article
• Language: English
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.5b00733

We provide a simple procedure for using inexpensive ab initio calculations to compute exchange coupling constants, J AB, for multiradical molecules containing both an arbitrary number of radical sites and an arbitrary number of unpaired electrons. For a system comprised of 2M unpaired electrons, one needs only to compute states having the Ŝ z quantum number M – 1. Conveniently, these are precisely the states that are accessed by the family of single spin–flip methods. Building an effective Hamiltonian with these states allows one to extract all of the J AB constants in the molecule. Unlike approaches based on density functional theory, this procedure relies on neither spin-contaminated states nor nonunique spin-projection formulas. A key benefit is that it is possible to obtain completely spin-pure exchange coupling constants with inexpensive ab initio calculations. A couple of examples are provided to illustrate the approach, including a 4-nickel cubane complex and a 6-chromium horseshoe complex with 18 entangled electrons.