  One of the major computational bottlenecks in one-body reduced density matrix
(1RDM) functional theory is the evaluation of approximate 1RDM functionals and
their derivatives. The reason is that more advanced approximate functionals are
almost exclusively defined in the natural orbital basis, so a 4-index
transformation of the two-electron integrals appears to be unavoidable. I will
show that this is not the case and that so-called separable functionals can be
evaluated much more efficiently, i.e. only at cubic cost in the basis size.
Since most approximate functionals are actually separable, this new algorithm
is an important development to make 1RDM functional theory calculations
feasible for large electronic systems.
