same_spin_doubles works incorrectly in UCCSD

Information

• Qiskit Aqua version: qiskit-master
• Python version: 3.8
• Operating system: Ubuntu

What is the current behavior?

same_spin_doubles = True in UCCSD variational form does not work correctly, when only beta same spin double excitations are required.

Steps to reproduce the problem

I faced this problem with triplet O2 molecule. For alpha orbitals there is only one unoccupied spin-orbital, while for beta there are three. I’m expecting to have same spin double excitations only for beta orbitals. However, there are no such excitations when same_spin_doubles = True.

What is the expected behavior?

Suggested solutions

Change condition here. When this condition is False due to alpha orbitals, beta сondition is omitted. My solution is:

        if same_spin_doubles:
            if len(active_occ_list_alpha) > 1 and len(active_unocc_list_alpha) > 1:
                for i, occ_alpha in enumerate(active_occ_list_alpha[:-1]):
                    for j, unocc_alpha in enumerate(active_unocc_list_alpha[:-1]):
                        for occ_alpha_1 in active_occ_list_alpha[i + 1:]:
                            for unocc_alpha_1 in active_unocc_list_alpha[j + 1:]:
                                double_excitations.append([occ_alpha, unocc_alpha,
                                                           occ_alpha_1, unocc_alpha_1])

            up_active_occ_list = active_occ_list_beta
            up_active_unocc_list = active_unocc_list_beta

            if len(active_occ_list_beta) > 1 and len(active_unocc_list_beta) > 1:        
                for i, occ_beta in enumerate(up_active_occ_list[:-1]):
                    for j, unocc_beta in enumerate(up_active_unocc_list[:-1]):
                        for occ_beta_1 in up_active_occ_list[i + 1:]:
                            for unocc_beta_1 in up_active_unocc_list[j + 1:]:
                                double_excitations.append([occ_beta, unocc_beta,
                                                           occ_beta_1, unocc_beta_1])

[MariaSapova]

@mrossinek Now it looks fine. I think the issue is solved. Thank you!

[mrossinek]

Alright I just pushed a fix and the only new excitations are now the following:

((5, 7), (6, 8))
((5, 7), (6, 9))
((5, 8), (6, 9))