# What Prob size is better for gmx SASA?

**URL:** <https://gromacs.bioexcel.eu/t/what-prob-size-is-better-for-gmx-sasa/2340>\
**Category:** User discussions\
**Tags:** gmx-sasa\
**Created:** [June 10, 2021, 11:12am UTC](https://gromacs.bioexcel.eu/t/what-prob-size-is-better-for-gmx-sasa/2340 "2021-06-10T11:12:00Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![mj.rezayani](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/mj.rezayani/32/3918_2.png) [@mj.rezayani](https://gromacs.bioexcel.eu/u/mj.rezayani)\
**Post date:** [June 10, 2021, 11:12am UTC](https://gromacs.bioexcel.eu/t/what-prob-size-is-better-for-gmx-sasa/2340/1 "2021-06-10T11:12:00Z")

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Dear all,

I want to calculate solvent accessible surface area using `gmx sasa`. The probe size is 0.14 nm by default. Do I have to change probe size to 0.315 ( sigma of Tip3p Oxygen)?  
When I changed it to 0.315 the average SASA is around zero. But, when I use default size it is about 200 nm^2. Which one is true?

Thanks alot

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**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [June 10, 2021, 1:24pm UTC](https://gromacs.bioexcel.eu/t/what-prob-size-is-better-for-gmx-sasa/2340/2 "2021-06-10T13:24:43Z")

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The actual radius of a water molecule is 0.14 nm, which is why that is the default and it is the value you should use. The value of σ is a force field-dependent quantity and represents the distance at which the LJ potential is zero between two atoms. It is not the size of the atom/molecule.