Strain control of spin and orbital transitions in La$_2$NiO$_4$

We have studied the electronic and magnetic structure of the layered nickelates, La$_2$NiO$_4$ within density functional theory. We show that biaxial strain induces a high spin to low spin transition, which coincides with a significant change in the $x^2-y$ and $3z^2-r^2$ orbital occupancy. We discuss the role of the on-site Coulomb interaction, the crystal field, and prospects for the strain control of the spin and orbital state.