is the lone pair, which arises from an in-phase combination of the N 2p z and the H 1s orbitals. The lowest unoccupied molecular orbital (LUMO) is the out-of-phase combination, and it also contains contributions from orbitals with higher principle quantum numbers. As a result, the a 1 symmetry LUMO is very diffuse and Rydberg-like, with a large tail on the hydrogen atoms (Figure ). When many ammonia molecules are placed near one another, oriented so that their hydrogen atoms point towards each other, "ammonia cluster" orbitals are made from a linear combination of these LUMOs. The lowest energy "cluster orbital" is one where all of the NH 3 LUMOs are in-phase, and it becomes filled when an excess electron is added to the cluster, resulting in a chemical species whose formula can be written as e -@(NH 3 ) n . An example of the resulting singly occupied molecular orbital (SOMO), obtained with DFT calculations for a tetrahedral cluster of four ammonia molecules (e -@(NH 3 ) 4 ), is illustrated in Figure . This SOMO can therefore be interpreted as a consequence of multi-centered-bond (MCB) formation from an overlap of the partially filled ammonia LUMOs.