Question about g

[Aleh Sudakou]

Dear Dr. Qianqian Fang,

Could you please clarify what happens when the optional variable "gscatter" is set to its default -1 value? (The maximum allowed number is 1e9)

Does -1 mean the g value is always ignored or never ignored?

"gscatter" is briefly explained in Section 1.7:

https://mcx.space/wiki/index.cgi?Doc/mcx_help#Additional_options  

Thanks!

Best wishes,

Aleh Sudakou

[Qianqian Fang]

hi Aleh,

there is a typo in the help page - the default value of gscatter is 1e9. it has been fixed.

it's restriction should be "can not be negative". this is because gscatter is defined as an "unsigned int", see source code here

https://github.com/fangq/mcx/blob/v2024.2/src/mcx_core.h#L187
https://github.com/fangq/mcx/blob/v2024.2/src/mcx_utils.c#L335

if you accidentally set g as -1 (0xFFFFFFFF), it is actually going to set the unsigned value to 0xFFFFFFFF=2^32=4.295e9, which is even higher than 1e9.

you can try it in the mcxlab examples, set cfg.gscatter=-1 and see the printed value.

either 1e9 or -1 won't ignore g, rather strictly following g in all scattering events.

if you want to ignore it, you should set gscatter to 0. This will completely ignore g, and use mus' = mus*(1-g) for all scattering events.

let me know if this makes sense to you.

Qianqian

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