Default: HF, HCl, HBr, HI, HAt
Neutral atoms: 1 bond is allowed.
Two of the ligands should be F, Cl or oxygen; and one more atom from the 14th, 15th, 16th columns.
Double bonded oxygen and one any other atom from 14th, 15th, 16th columns are also accepted.
F-, Cl-, Br-, I-, At- are accepted.
1+ charged ions: 2 filled valences are necessary (bonds or radicals). If absent, implicit Hydrogen atoms added.