Hello fellow Ifeffiters This a story about making feff.inp input files for FEFF. The more experienced readers of this list will probably be well aware of the issues involved, but I thought that relative newcomers such as myself might not know (which is why I'm posting this on the Ifefft list rather than the Feff list). In a nutshell, it is wise to create a large cluster of atoms to put into Feff (ie use a large Rmax in Atoms), but limit the number of paths with a reduced Rmax in Feff. I used Artemis/Atoms to make up a Feff input file (feff.inp) from crystallographic parameters, setting Rmax in Atoms to a relatively large value of 7.5 Å to create quite a large cluster of atoms. I then used FEFF to calculate scattering paths within the cluster, but first reduced the Rmax in feff.inp to 5.5 Å to limit the number of paths. So far so good. I then (for reasons too involved to discuss here) edited the feff.inp file to delete all atoms more than 5.5 Å from the centre, and ran Feff again. The results (judging from the relative amplitude values) were different, and in the case of some paths very different. Some intense paths changed in amplitude by a factor of 2 or greater, although I haven't looked at the finer details of the Feff results. The paths that changed the most were high-order multiple scattering paths, and surprisingly those paths did not involve atoms at the edges of the 5.5 Å radius. Now I'm sure that someone else can explain the reasons for this much better than I can, but I assume that the potentials for the atoms outside the 5.5 Å radius (which were neglected in the second calculations) significantly affect those inside the radius. I know that its always been my practice to perform the Feff calculation using only those atoms that I think are contributing to my spectrum, and I'm sure that others do the same. My advice is to use a larger cluster of atoms in Feff if you can - but as to how large I might leave that to someone else to comment. Regards, Peter PS - and four cheers to Bruce for improvements to Artemis, I can't wait for the Windows update. Peter Southon Post Doctoral Fellow School of Chemistry University of Sydney NSW 2006, Australia +61 2 9351 4425
Peter, Your observations are correct. I always run the feff calculation multiple times, adding atoms to the feff input file each calculation until the results converge. Jeff On May 20, 2004, at 8:15 PM, Peter Southon wrote:
Hello fellow Ifeffiters
This a story about making feff.inp input files for FEFF. The more experienced readers of this list will probably be well aware of the issues involved, but I thought that relative newcomers such as myself might not know (which is why I'm posting this on the Ifefft list rather than the Feff list). In a nutshell, it is wise to create a large cluster of atoms to put into Feff (ie use a large Rmax in Atoms), but limit the number of paths with a reduced Rmax in Feff.
I used Artemis/Atoms to make up a Feff input file (feff.inp) from crystallographic parameters, setting Rmax in Atoms to a relatively large value of 7.5 Å to create quite a large cluster of atoms. I then used FEFF to calculate scattering paths within the cluster, but first reduced the Rmax in feff.inp to 5.5 Å to limit the number of paths. So far so good.
I then (for reasons too involved to discuss here) edited the feff.inp file to delete all atoms more than 5.5 Å from the centre, and ran Feff again.. The results (judging from the relative amplitude values) were different, and in the case of some paths very different. Some intense paths changed in amplitude by a factor of 2 or greater, although I haven't looked at the finer details of the Feff results. The paths that changed the most were high-order multiple scattering paths, and surprisingly those paths did not involve atoms at the edges of the 5.5 Å radius.
Now I'm sure that someone else can explain the reasons for this much better than I can, but I assume that the potentials for the atoms outside the 5.5 Å radius (which were neglected in the second calculations) significantly affect those inside the radius. I know that its always been my practice to perform the Feff calculation using only those atoms that I think are contributing to my spectrum, and I'm sure that others do the same. My advice is to use a larger cluster of atoms in Feff if you can - but as to how large I might leave that to someone else to comment.
Regards, Peter
PS - and four cheers to Bruce for improvements to Artemis, I can't wait for the Windows update.
Peter Southon Post Doctoral Fellow School of Chemistry University of Sydney NSW 2006, Australia +61 2 9351 4425
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participants (2)
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Jeff Terry -
Peter Southon