Updated H_H length from 0.6 to 0.75 as defined in CCCBDB fro [H][H] (#829)

at 0.6, [H][H] conformer generated by RDKIT is seen as not isomorphic

Author: alongd

arc/common.py

@@ -625,7 +625,7 @@ def get_element_mass(input_element: Union[int, str],
                       'O_O': 1.48, 'O_S': 1.67, 'O_Si': 1.66, 'O_H': 0.96, 'O_F': 1.24, 'O_Cl': 1.65, 'O_Br': 1.80, 'O_I': 2.00,
                       'S_S': 2.05, 'S_Si': 2.18, 'S_H': 1.34, 'S_F': 1.55, 'S_Cl': 2.07, 'S_Br': 2.15, 'S_I': 2.34,
                       'Si_Si': 2.35, 'Si_H': 1.48, 'Si_F': 1.75, 'Si_Cl': 2.03, 'Si_Br': 2.31, 'Si_I': 2.50,
-                      'H_H': 0.60, 'H_F': 0.92, 'H_Cl': 1.27, 'H_Br': 1.41, 'H_I': 1.63,
+                      'H_H': 0.75, 'H_F': 0.92, 'H_Cl': 1.27, 'H_Br': 1.41, 'H_I': 1.63,
                       'F_F': 1.42, 'F_Cl': 1.57, 'F_Br': 1.72, 'F_I': 1.91,
                       'Cl_Cl': 1.99, 'Cl_Br': 2.13, 'Cl_I': 2.32,
                       'Br_Br': 2.29, 'Br_I': 2.47,