VSEPR theory
In the vapor phase the BaF2 molecule is non-linear with an F-Ba-F angle of approximately 108°. Its nonlinearity violates VSEPR theory. Ab initio calculations indicate that contributions from d orbitals in the shell below the valence shell are responsible. Another proposal is that polarisation of the electron core of the barium atom creates an approximately tetrahedral distribution of charge that interacts with the Ba-F bonds.