Full configuration interaction in the space of fractionally occupied unrestricted Hartree–Fock natural orbitals is proposed as an inexpensive alternative to the complete active space–self-consistent-field method for ground states. In most cases, this method requires only a fraction of the computational effort of the latter and gives comparable results, as demonstrated on several examples. Gradient evaluation, while more complex in formulation, is computationally not significantly more expensive than unrestricted Hartree–Fock gradients.

The performance of density-functional theory to solve the exact, nonrelativistic, many-electron problem for magnetic systems has been explored in a new implementation imposing space and spin symmetry constraints, as in ab initio wave function theory. Calculations on selected systems representative of organic diradicals, molecular magnets and antiferromagnetic solids carried out with and without these constraints lead to contradictory results, which provide numerical illustration on this usually obviated problem. It is concluded that the present exchange-correlation functionals provide reasonable numerical results although for the wrong physical reasons, thus evidencing the need for continued search for more accurate expressions.