  A two reference determinant version of the distinguishable cluster with
singles and doubles (DCSD) has been developed. We have implemented the two
determinant distinguishable cluster (2D-DCSD) and the corresponding traditional
2D-CCSD method in a new open-source package written in Julia called ElemCo.jl.
The methods were benchmarked on singlet and triplet excited states of valence
and Rydberg character, as well as for singlet-triplet gaps of diradicals. It is
demonstrated that the distinguishable cluster approximation improves the
accuracy of 2D-CCSD.
