Like (CO)4, Do (CS)4 and (CSe)4 have a triplet ground state?

Cyclobutane-1,2,3,4-tetraone, (CO)4, was computationally predicted and, subsequently, experimentally confirmed to have a triplet ground state, in which a b2g σ MO and an a2u π MO were each singly occupied. In contrast, the (U)CCSD(T) calculations reported herein found that cyclobutane-1,2,3,4-tetrathione, (CS)4, and cyclobutane-1,2,3,4-tetraselenone, (CSe)4, both had singlet ground states, in which the b2g σ MO was doubly occupied and the a2u π MO was empty. Our calculations showed that both the longer C=X distances and smaller coefficients on the carbon atoms in the b2g and a2u MOs of (CS)4 and (CSe)4 contributed to the difference between the ground states of these two molecules and the ground state of (CO)4. An experimental test of the prediction of a singlet ground state for (CS)4 is proposed.