Dear Tian,
Your kind attention for assigning your valuable time to provide such an informative explanation is highly appreciated.

Sincerely,
Saeed

Dear Saeed,

Using fragment 1 with alpha unpaired electron and fragment 2 with beta unpaired electron is just a convention. You can also exchange the setting.

Best,

Tian

Dear Tian,
A nice and informative example has been analyzed in the sobEDA manual regarding and open-shell system namely CH3-NH2. To define corresponding fragments CH3 and NH2 have been considered as doublet radicals (associated with a homiletic cleavage of C-N bond). On the other hand, the only unpaired electron over CH3 is considered as alpha electron while that of NH2 as beta electron. If we change these multiplicities, i.e. beta for CH3 and alpha for NH2, a quite same results would be obtained for sobEDA. Could you please let me know is there any specific reason you have considered alpha electron for CH3 and beta electron for NH2?

Yours sincerely,
Saeed