# How to set sel ?

`sel` is short for "selected number of atoms in `rcut`".

`sel_a[i]` is a list of integers. The length of the list should be the same as the number of atom types in the system. 

`sel_a[i]` gives the number of selected number of type `i` neighbors within `rcut`. To ensure that the results are strictly accurate, `sel_a[i]` should be larger than the largest number of type `i` neighbors in the `rcut`.

However, the computation overhead increases with `sel_a[i]`, therefore, `sel_a[i]` should be as small as possible.

The setting of `sel_a[i]` should balance the above two considerations.