The method of choice for the treatment of the excited states of AB is EOM-EE, however, using this method for the pAB molecule is complicated by its open-shell character, which can lead to small or severe spin-contamination . Generally, states derived by excitations to or from open-shell orbitals are well-behaved whereas the states derived by excitations from doubly occupied to unoccupied orbitals are poorly described because of the spin-incompleteness of the EOM-EE-CCSD ansatz (this problem can be remedied by including selected triple excitations 139 ).