  The harmonium model has long been regarded as an exactly solvable laboratory
bench for quantum chemistry [Heisenberg, 1926]. For studying correlation
energy, only the ground state of the system has received consideration
heretofore. This is a spin singlet state. In this work we exhaustively study
the lowest excited (spin triplet) harmonium state, with the main purpose of
revisiting the relation between entanglement measures and correlation energy
for this quite different species. The task is made easier by working with
Wigner quasiprobabilities on phase space.
