[Wien] how to calculate the energy of single molecule(i.e. O2)?

[yxl@email.jlu.edu.cn]

Dear users,
         how to calculate the energy of single molecule(i.e. O2)? how to define  the distance of two oxygen atom,according to the bond length? Do i should minimize the energy or spin-polarize calculation? and why? Please help me , i have thinked about it for a few days.Thank you in advance!
                                                                         yours sincerely,
                                                                                           hongxia