spherical harmonics for anisotropic peak shapes

danweber

Hi all,

I am trying to capture anisotropic peak broadening by applying spherical harmonics in Topas 6. The initial code I use is based on the example from Andreas Leineweber's chapter in the Topas book, see below.

When I start with a zeta of 1, I get an error message "Unhandled exception in calc_peaks_in_buffer". If I set zeta to 0.5, the code refines and gets loaded into the inp, with zeta refining to 1 and some of the SH parameters are negative. When I try to run the code again, the I get the "Unhandled exception in calc_peaks_in_buffer" message again.

Does anyone have a resolution for this issue?

Many thanks,
Dan

Code:
prm D_0 50
spherical_harmonics_hkl A_hkl
sh_order 4
prm D_hkl = Max(D_0 A_hkl, 1);
prm zeta 0.5 min 0 max 1
prm D_hklG = D_hkl/(1-zeta);
prm D_hklL = D_hkl/zeta;
CS_G(D_hklG)
CS_L(D_hklL)
prm D_volhkl = D_hkl/Voigt_Integral_Breadth_GL ((1-zeta), zeta);

johnsoevans

One possibility might be that your zeta is hitting 1.0 and you then divide by zero. Perhaps try limiting to 0.99 as a test?

Another problem with anisotropic peak shapes is that they can cause the fwhm to go negative. For example if you look in the Stephens peak shape macros you'll see a line like the one below to prevent this.

local #m_unique pp_ = D_spacing² * Sqrt(Max(mhkl,0)) Tan(Th).......

In your case the Max(D_0 A_hkl, 1); should be doing the same thing. You could try adding something in the CS_G macros like Min(D_hklG, 0) to help.

danweber

Hi John,

thanks for the hint with zeta hitting zero. Setting the min and max values to 0.01 and 0.99 did the trick and refinements are running now.

Best, Dan