Re: ? about l_scmt etc.
Matt & IFEFFIT group, This is actually a feff.inp question.... but I'm hoping someone might be able to clarify this for me... I was wondering what appropriate values are for l_scmt, l_fms, and stoichiometry for the feff.inp I'm working with regarding Au on a Ni surface (I'm fitting with feff calculations with either a single Au atom in the first Ni layer or two Au atoms in the first Ni layer)? Should they all be the same for each potential? I just wanted to check if my values were appropriate. I've used some values based off of an example file all along... but I'm not sure if I used them appropriately for my scenerio. I've read the FEFF8.10 manual but I'm still unclear as to the proper values... (i.e. in the past I have used the following for 2 Au's in the surface) POTENTIALS * ipot z element l_scmt l_fms stoichiometry 0 79 Au 3 3 0.001 1 28 Ni 2 2 4 2 79 Au2 3 3 4 Oh... one more question... I haven't been using the ellipticity card in the feff.inp file, even though I've been using "POLARIZATION 1 0 0"... is this OK? Thanks, Kristine
Hi Kristine,
This is actually a feff.inp question.... but I'm hoping someone might be able to clarify this for me...
I hope someone can provide better insight for Feff8, but here's what I'd say:
I was wondering what appropriate values are for l_scmt, l_fms, and stoichiometry for the feff.inp I'm working with regarding Au on a Ni surface (I'm fitting with feff calculations with either a single Au atom in the first Ni layer or two Au atoms in the first Ni layer)?
Should they all be the same for each potential?
For EXAFS and EXAFS fitting, the FMS and XANES calculations should not be done. For EXAFS, these values don't matter (I think!). For what it's worth, L_fms basically sets how high in k the FMS approach works well enough to use. It's typically a number <~ 5. I'm not sure what l_scmt is...
(i.e. in the past I have used the following for 2 Au's in the surface) POTENTIALS * ipot z element l_scmt l_fms stoichiometry 0 79 Au 3 3 0.001 1 28 Ni 2 2 4 2 79 Au2 3 3 4
Oh, I think those might be ipot z element lmin lmax and that lmin=-1, lmax=3 are reasonable default values. Hopefully someone can clarify this....
Oh... one more question... I haven't been using the ellipticity card in the feff.inp file, even though I've been using "POLARIZATION 1 0 0"... is this OK?
Yes, the ellipticity card is for describing polarizations that are not linear, such as for magnetic circular dichroism. For your surface EXAFS experiments at APS 5-ID, the x-rays were highly linearly polarized. Hope that helps, --Matt
Hi Kristine, Matt, I just remembered a discussion on the (meanwhile rather inactive?!) feffusers list concerning ELLIPTICITY and thought I should quote that.
Oh... one more question... I haven't been using the ellipticity card in the feff.inp file, even though I've been using "POLARIZATION 1 0 0"... is this OK?
Yes, the ellipticity card is for describing polarizations that are not linear, such as for magnetic circular dichroism. For your surface EXAFS experiments at APS 5-ID, the x-rays were highly linearly polarized.
As of 2002-04-25 Alex Ankudinov wrote: - - - - snip - - - - ELLIPTICITY specifies the direction of incoming x-ray and the ratio of intensities along two perpendicular directions, one of which is specified by POLARIZATION card and the other is found automatically. ELLIPTICITY is not intended to be used for circular polarized light. One has to use XMCD card, which assumes 100% circular polarized light. - - - - snip - - - - Best, Peter -- -------------------------------------------------------------- Peter Pfalzer Universitaet Augsburg Tel: +49-821-598-3215 Lehrstuhl fuer Experimentalphysik II Fax: +49-821-598-3411 Universitaetsstr. 1 D-86135 Augsburg Germany Peter.Pfalzer@physik.uni-augsburg.de http://www.physik.uni-augsburg.de/exp2 --------------------------------------------------------------
For EXAFS and EXAFS fitting, the FMS and XANES calculations should not be done. For EXAFS, these values don't matter (I think!). For what it's worth, L_fms basically sets how high in k the FMS approach works well enough to use. It's typically a number <~ 5. I'm not sure what l_scmt is...
I agree that the FMS and XANES calculations need not be done if you are interested in doing EXAFS analysis.
(i.e. in the past I have used the following for 2 Au's in the surface) POTENTIALS * ipot z element l_scmt l_fms stoichiometry 0 79 Au 3 3 0.001 1 28 Ni 2 2 4 2 79 Au2 3 3 4
Oh, I think those might be ipot z element lmin lmax
and that lmin=-1, lmax=3 are reasonable default values. Hopefully someone can clarify this....
That's not correct. "scmt" = "self consistent muffin tin" and "fms" = "full multiple scattering". Elsewhere in the feff-iverse, scmt is referred to as "scf" or self-consistent field. The l_scmt value is used in the calculation of the self-consistent potentials. The l_fms value is used in the calculation of the final densities of state and xanes. Each one limits the size of the angular momentum potion of the basis used in the matrix algebra required to do the feff8 calculation. The reason there are two is to allow the possibility of considering different sized bases while determining the potentials than while computing the final xanes spectrum. There are some reasons why one might want that freedom. In practice, one can usually use the same values for l_scmt and l_fms. For first row elements, l=1 is often fine. If one has the time and computer power, l=2 might result in a better calculation. For transition metals, l=2 is a good choice. For heavier things, l=3 should be used. Heavier elements are likely to have f-electrons in the valence band or low-lying f-states, thus a value of 3 is necessary. An l-value of -1 tells feff to use its default for that element. The defaults are more-or-less as I explained in the last paragraph. Thus, the easiest way to use feff8 is to use l=-1 for every atom and for both scmt and fms. B
participants (4)
-
Bruce Ravel -
k-kupiecki＠northwestern.edu -
Matt Newville -
Peter Pfalzer