Localized orbital theory and ammonia triborane

In a previous paper [J. Subotnik, Y. Shao and W. Liang, and M. Head-Gordon, J. Chem. Phys., 2004, 121, 9220], we proposed a new and efficient method for computing localized Edmiston-Ruedenberg (ER) orbitals, which are those localized orbitals that maximize self-interaction. In this paper, we improve upon our previous algorithm in two ways. First, we incorporate the resolution of the identity (RI) and atomic resolution of the identity (ARI) approximations when generating the relevant integrals, which allows for a drastic reduction in computational cost. Second, after convergence to a stationary point, we efficiently calculate the lowest mode of the Hessian matrix in order to either (i) confirm that we have found a minimum, or if not, (ii) move us away from the current saddle point. This gives our algorithm added stability. As a chemical example, in this paper, we investigate the electronic structure (including the localized orbitals) of ammonia triborane (NH₃B₃H ₇). Though ammonia triborane is a very electron-deficient compound, it forms a stable white powder which is now being investigated as a potential hydrogen storage material. In contrast to previous electronic structure predictions, our calculations show that ammonia triborane has one localized molecular orbital in the center of the electron-deficient triborane ring (much like the single molecular orbital in H₃⁺), which gives the molecule added energetic stability. Furthermore, we believe that NH ₃B₃H₇ is the smallest stable molecule supporting such a closed, three-center BBB bond.