Embedding tools to improve density functionals.*

As is well known, standard approximations to the exchange-correlation (XC) functional often do not yield accurate energies and/or spin-densities when chemical bonds are stretched. We use density embedding theory to examine the behavior of the non-additive contribution to the XC energy and to propose physically-motivated approximations for this contribution as atoms dissociate. We also discuss a recent approach to develop improved approximations for the non-additive non-interacting kinetic energy, and why this is a challenging problem.