| Formally, the relativistic theory of X-ray absorption for nonmagnetic materials looks the same as non-relativistic theory. Usually relativistic effects are expected to be important for high Z elements, but even for low Z magnetic materials relativity is essential to describe several spin-dependent effects, while the non-relativistic theory gives null results. A new efficient way is suggested to treat relativistic and spin effects simultaneously. This approach is tested in calculations for several materials.; In order to achieve good agreement between x-ray absorption calculations and experiment, a good self-energy model is necessary. The self-energy or exchange correlation potential is the most important many-body correction for the calculation of x-ray absorption. The x-ray magnetic circular dichroism (XMCD) signal is primarily due to the spin dependence of the self-energy. A number of ways to improve the self-energy calculations are discussed and tested. The edge specific sum rules are also discussed. A new derivation of these sum rules is proposed. Many body analysis within sudden approximation is done, and shows that the sum rules give ground state properties.; A new version of the ab initio XAS code, FEFF7, has been developed, in which a Dirac-Fock atomic code replaces the LDA atomic code. This gives a better description of the core electrons, and also allows one to test the importance of the self-energy nonlocality. A new fully relativistic subroutine has been written for dipole matrix elements. Spin dependence of exchange-correlation potential is included. Results for a number of materials are discussed. |
