miscellaneous comments

[JisuJung928]

SIMPLE-NN_v2/test_sf/test_sf.py

Line 88 in 14b4c14

def _g4_ker(dist_ij,dist_ik,dist_jk, cutoff,eta ,zeta ,lamda , angle = False):

For readability, please define angular ACSF including 2**(1-zeta) as it is written (J. Chem. Phys. 134, 074106 (2011)).

SIMPLE-NN_v2/test_sf/test_sf.py

Line 18 in 14b4c14

import math

How about replacing all functions of math with numpy for dtype management and a faster matrix calculation someday?

SIMPLE-NN_v2/symmetry_function/__init__.py

Line 103 in fde2d08

res = self._init_result( type_num , structure_names , structure_weights , idx , atom_i)

How about dealing with res variable in inline for avoiding overhead from calling the function in python?

[Nanco-L]

Overhead from calling the function is good point that we need to check.
In addition, I suggest to avoid writing code in __init__.py.
(I know that I wrote the code in features/symmetry_function/__init__.py for the current version of SIMPLE-NN.. But it is not recommended.)