While quantum chemical predictions have strongly suggested a decade ago the existence of mercury in its oxidation state +IV, no experimental evidence has been found yet. To enable the search for alternative targets and preparation routes by quantum chemical methods, the present work has validated density functional methods against accurate CCSD(T) results for structures, reaction energies and activation barriers for X₂-elimination, and atomization energies for three HgX₄ systems (X = F, Cl, H). Hybrid functionals with ca. 20% Hartree–Fock exchange like B3LYP, B1LYP or MPW1PW91 have provided the best energetics, whereas local or gradient-corrected “pure” functionals overestimate, and the BHandHLYP hybrid functional underestimates the stability of the Hg^IV state. Basis sets are suggested that provide a reasonable compromise between accuracy and computational effort in calculations on larger systems.