For those with pressing requirements to calculate non-MT XANES, I have collected here some brief notes about possible alternatives. I am restricting attention here to methods which can calculate XANES, taking into account the full details of the cluster or crystal potential. Some I had already heard of, but others have been brought to light by perusal of the web pages of colleagues whom I haven't seen for a while (notably C Brouder and Y Joly). I am not sure that all of them are available to the general public (if not, it is possible that their authors may be amenable to proposals of collaboration).

It should be noted that even periodic-system codes may be used for cluster calculations, by using supercell methods, although these tend to become cumbersome quite quickly. Sometimes it is not even necessary (for reasons which are not entirely clear) to take into account the core hole and even supercell methods are not needed.

Some possibilities which are based on multiple-scattering or related methods include:

 

• FDMNES: Implemented by Yves Joly (CNRS Crystallography Laboratory, Grenoble) and others, this is a hybrid of multiple-scattering and grid methods; and (I believe) follows the idea that Rino Natoli suggested in 1992, namely, to reduce the atomic spheres in size so that the potential is essentially spherically symmetric, solving Schrödinger's equation in the (now larger) interstitial region on a (cubic) grid by standard finite-difference methods. I had a chance to discuss some of this work with Yves at the XAFS 12 conference (Malmö, 2003). I gather that the method works well, although the interstitial region solution (with a volume-related scaling behaviour) appears to be a bottleneck, limiting the size of possible systems to be treated. Most recently, I understand, a version of the program is being worked on by my former Scisoft colleague, Alessandro Mirone, which may address this issue: I look forward to hearing more of this.
• SPRKKR: A spin-polarized relativistic KKR code for periodic systems developed by Hubert Ebert (Ludwig-Maximilians-University, Munich) and colleagues. By chance I met Prof Ebert in 2006 and had a brief conversation about non-MT methods. I understood that a non-MT version of SPRKKR has become (or is soon to be) available, based on a variant of the cellular KKR method mentioned above.
• FEFF: This well-known MT-based multiple-scattering code for EXAFS, due to John Rehr (University of Washington, Seattle, USA) and various co-workers, has in recent years been extended to XANES, with an implementation of the MSW method, and can use SCF potentials. I learned from John in 2002 that they were developing a non-MT capability (based on "fuzzy polyhedra" - if I remember correctly), and were in the process of trying it on some existing benchmarks (including our Cl2 results mentioned above). At the XAFS 12 conference A Ankudinov spoke about general developments in FEFF, including the interesting results of this testing. As of current writing I do not know whether it has been included in the public release.

 

Non-multiple-scattering periodic-system codes:

• PARATEC: This pseudo-potential code (or a modified version) has been used by Delphine Cabaret (IMPMC, Université Pierre et Marie Curie - Paris 6) and co-workers to produce some impressive results that were presented at the XAFS 12 conference. I understand from Delphine that supercell methods with these codes are much less cumbersome than one might expect, and look forward to hearing more. (If she recalls our conversation, I am still interested to see what PARATEC makes of the Cl2 benchmark(?...).)
• WIEN 2000: A well-known Full-Potential LAPW code due to P Blaha and K Schwarz et al (Technical University, Vienna) for various types of calculations in solid-state systems. In Foulis, Pettifer and Jennings (2002) we present some results for TiO2 (calculated in fact with Wien 1987 by Vince Jennings) whose good agreement with experiment rather surprised us, considering that a neutral potential (no core hole) was used for the final-state potential.

 

More speculatively:

• MXAN: This is a XANES fitting code due to Maurizio Benfatto and Stefano Della Longa (although a variety of other people have worked on it, including yours truly). While the MSW part is a descendant of Rino Natoli's CONTINUUM code and therefore muffin-tin based, there is an intention to upgrade the atomic part of the calculation to include higher multipoles in the potential. Given that the code makes use of overlapping atomic spheres, this may go some way towards correcting the MT problem.

 

If I find the time, I shall dig around some more and perhaps expand this list, as well as trying to find the definitive references and/or web links for the entries. As I mentioned in the Foreword I make no attempt to evaluate any of them. In fact, I am only passingly familiar with a couple, but the fact that I mention them here is intended to suggest that they are worth considering.