sentences:  The Greek letter φ in their name refers to f orbitals, since the orbital symmetry of the φ bond is the same as that of the usual (6-lobed) type of f orbital when seen down the bond axis.There was one possible candidate known in 2005 of a molecule with phi bonding (a U−U bond, in the molecule U2).However, later studies by the ETH Zurich that accounted for spin orbit interactions found that the bonding was only of fourth order.