Early experimental investigations on spin-carrying metal centers, initially aimed at understanding how nature converts water to hydrogen and fixes nitrogen, formed the basis for the field of molecular magnetism 25 years ago. Shortly thereafter such molecules were postulated to be potentially realistic physical manifestations of spin-based qubits for quantum computers [1]. In this talk I will discuss computational and theoretical challenges associated with the accurate quantum-mechanical computational description of these systems and discuss two recent computer experiments that respectively: (A) show how a low-energy quantum-sensing technique can be used to deduce the chemical splitting of water into hydroxyl and hydrogen molecules [2] and (B) demonstrate how computational density-functional-based methods can be used to accurately determine the properties and complexities of putative molecular magnetic qubits that are composed of a perfect triangle of half-integer spin metal ions[3,4]. Connections to recent experimental publications will be made.

The Mn₁₂O₁₂(COOR)₁₆(HOH)₄ molecule, with S₄ symmetry, has four of everything. Our recent calculations find that this system readily accepts four excess electrons at the cost of only 0.32 eV in vacuum. This molecule exhibits a macro-spin with S=10. It has received significant past interest due to the experimental observations and theoretically confirmed process of quantum tunneling of magnetization (QTM). Here, we show that the spectroscopic signatures associates with QTM are extremely sensitive to the presence of the four HOH terminators (e.g. 4 waters vs. 2H₂ and 2OH) and to the number of added electrons (0 vs. 4). Our calculations suggest that QTM can be used as an ultra-low-energy non-destructive observation of water decomposition in a molecule with a core Mn₄O₄ unit that bears a striking similarity to the reaction center in the oxygen evolving complex. See Ref. 1.

Recently, Boudalis et al have experimentally observed the magneto-electric effect in a chiral Fe₃O(NC₅H₅)₃(O₂CC₆H₅)₆ molecule [5] and have noted further that this is the first possible spin-electric system based upon spin 5/2 metal centers. Our results [3], using standard density-functional methods, show that the spin-electric behavior of this molecule could be even more interesting as there are energetically competitive reference states with high and low local spins (S=5/2 vs. S=1/2) on the Fe³⁺ ions. We provide predictions of magnetic and x-ray spectroscopies to deduce the presence of both states. Possible uses for low-temperature quantum sensing of fields and pressure variations are suggested.

Recent efforts at improving standard approximations of density-functional theory using a new version of self-interaction-corrected DFT will be discussed within the context of this work [2,6].

[1] B. Georgeot and F. Mila, Chirality of triangular antiferromagnetic clusters as a qubit, Phys. Rev. Lett. 104, 200502 (2010).

[2] J. Batool, T. Hahn and M.R. Pederson, Magnetic Signatures of Hydroxyl and Water Terminated Neutral and Tetra-anionic Mn₁₂-Acetate, J. Comput. Chem. 25, 2301-2308 (2019).

[3]M. F. Islam, J. F. Nossa, C. M. Canali and M. Pederson, First-principles study of spin-electric coupling in a Cu₃ single molecular magnet, Phys. Rev. B 82 155446 (2010).

[4]A. I. Johnson, M. F. Islam, C. M. Canali and M. R. Pederson, A Multiferroic molecular magnetic qubit, Submitted to J. Chem. Phys. (https://arxiv.org/abs/1909.08803).

[5]A. K. Boudalis, J. Robert & P. Turek, 1^st demonstration of magnetoelectric coupling in a polynuclear molecular nanomagnet via EPR studies Fe₃O(O₂CPh)₆(Py)₃ClO₄, Chem. Eur. J 24 14896-14900 (2018).

 [6] M.R. Pederson, A Ruzsinszky and J. P. Perdew, Communication: Self-Interaction Correction with Unitary Invariance in Density Functional Theory, J. of Chem. Phys., 140, 121103 (2014).