Why is ozone O₃ diamagnetic? Please explain how to derive the magnetism  of polyatomic molecules? If we derive it using an addition of an oxygen atom to molecular oxygen ($\ce{O2}$) it will come to be paramagnetic like $\ce{O3}$?
Ozone's ground state is a singlet.  All electrons therein are paired,




However,

http://link.springer.com/article/10.1134%2FS0018143906040060#page-1 
http://link.springer.com/article/10.1134%2F1.1870062#page-1