X-ray constrained unrestricted Hartree-Fock and Douglas-Kroll-Hess wavefunctions.

The extension of the X-ray constrained (XC) wavefunction approach to open-shell systems using the unrestricted Hartree-Fock formalism is reported. The XC method is also extended to include relativistic effects using the scalar second-order Douglas-Kroll-Hess approach. The relativistic effects on the charge and spin density on two model compounds containing the copper and iron atom are reported. The size of the relativistic effects is investigated in real and reciprocal space; in addition, picture-change effects are investigated and discussed for the isolated Cu atom. It is found that the relativistic terms lead to changes in the densities that are much smaller than those from the X-ray constraint. Nevertheless, the use of the relativistic corrections in the ab initio model always leads to an improvement in the agreement statistics. An interesting result of the unrestricted XC technique is the possibility of obtaining experimentally derived spin densities from X-ray data.