Knowledge Bank

A number of ab initio approaches have been used to determine the equilibrium structures, energies, and harmonic vibrational frequencies of $N2H2$ in the ground state and in the excited singlet and triplet ($n,\pi \ast$) states. The methods include restricted (RHF) and unrestricted (UHF) Hartree-Fock, multiconfigurational self-configurational self-consistent field (MCSCF), single-reference (SRCI) and multireference (MRCI) configuration interaction including all single and double excitations from the reference configurations, and second-order Moller-Plesset perturbation theory (MP2) using RHF and UHF orbitals for the ground and excited states, respectively. Unlike the singlet excited state, for which broken-symmetry solutions were found at the RHF level, no symmetry breaking was encountered for the triplet state. The ground state MP2 structure of $N2H2$, which has $C2h$ symmetry, is in excellent agreement with the experimentally determined structure. The excited states are predicted to have nonplaner $C2$ structures with dihedral angles ranging from 96 to $106\circ$ for the triplet state and from 105 to $121\circ$ for the singlet. Except for the SRCI singlet adiabatic excitation energy, the effect of configuration interaction is to increase the vertical and adiabatic excitation energles of both excited states relative to the RHF values in single-reference calculations, and to decrease these excitation energies relative to the MCSCF values in multireference calculations, bringing the single-reference and multireference values into better agreement with each other. The MRCI vertical excitation energies are 2.6 eV for the triplet and 3.6 eV for the singlet, while the corresponding 0-0 transition energies are 1.9 and 2.9 eV, respectively.