  A benchmark ab initio study on the thermochemistry of the trans-HNNH,
cis-HNNH, and H$_2$NN isomers of diazene has been carried out using the CCSD(T)
coupled cluster method, basis sets as large as $[7s6p5d4f3g2h/5s4p3d2f1g]$, and
extrapolations towards the 1-particle basis set limit. The effects on
inner-shell correlation and of anharmonicity in the zero-point energy were
taken into account: accurate geometries and anharmonic force fields were thus
obtained as by-products. Our best computed $\Delta H^\circ_{f,0}$ for
trans-HNNH, 49.2 \pm 0.3 kcal/mol, is in very good agreement with a recent
experimental lower limit of 48.8 \pm 0.5 kcal/mol. CCSD(T) basis set limit
values for the isomerization energies at 0 K are 5.2 \pm 0.2 kcal/mol
(cis-trans) and 24.1 \pm 0.2 kcal/mol (iso-trans). Our best computed geometry
for trans-HNNH, $r_e$(NN)=1.2468 \AA, $r_e$(NH)=1.0283 \AA, and
$\theta_e$=106.17$^\circ$, reproduces the precisely known ground-state
rotational constants of trans-HNNH to within better than 0.1 %. The
rotation-vibration spectra of both cis-HNNH and H$_2$NN are dominated by very
strong Coriolis and Fermi resonances. In addition, the NH stretches in H$_2$NN
are so strongly anharmonic that vibrational perturbation theory breaks down,
and the molecule appears to be an excellent test case for variational
treatments of the vibrational Schr\"odinger equation.
