  Wavefunctions restricted to electron pair states are promising models for
strongly-correlated systems. Specifically, the pair Coupled Cluster Doubles
(pCCD) ansatz allows us to accurately describe bond dissociation processes and
heavy-element containing compounds with multiple quasi-degenerate
single-particle states. Here, we extend the pCCD method to model excited states
using the equation of motion (EOM) formalism. As the cluster operator of pCCD
is restricted to electron-pair excitations, EOM-pCCD allows us to target
excited electron-pair states only. To model singly excited states within
EOM-pCCD, we modify the configuration interaction ansatz of EOM-pCCD to contain
also single excitations. Our proposed model represents a simple and
cost-effective alternative to conventional EOM-CC methods to study singly
excited electronic states. The performance of the excited state models is
assessed against the lowest-lying excited states of the uranyl cation and the
two lowest-lying excited states of all-trans polyenes. Our numerical results
suggest that EOM-pCCD including single excitations is a good starting point to
target singly excited states.
