A number ofabinitioapproaches have been used to determine the equilibrium structures, energies, and vibrational frequencies of N2H2in the ground state and in the excited singlet and tripletnrarr;pgr;ast; states. The methods included restricted Hartreendash;Fock (RHF) and unrestricted Hartreendash;Fock (UHF), multiconfiguration selfhyphen;consistent field (MCSCF), singlehyphen;reference configuration interaction (SRCI) and multireference configuration interaction (MRCI) including all single and double excitations from the reference configurations, and secondhyphen;order Moslash;llerndash;Plesset perturbation theory (MP2) using RHF and UHF orbitals for ground and excited states, respectively. Unlike the singlet excited state, for which brokenhyphen;symmetry solutions were found at the RHF level, no symmetry breaking was encountered for the triplet state. The groundhyphen;state MCSCF and MP2 structures of N2H2, which haveC2h(transhyphen;planar) symmetry, are in good agreement with the experimental structure. The excited states are predicted to have nonplanarC2structures with dihedral angles ranging from 96deg; to 106deg; for the triplet state and from 105deg; to 121deg; for the singlet state. Except for the SRCI singlet adiabatic excitation energy, the effect of configuration interaction is to increase the vertical and adiabatic excitation energies of both excited states relative to the RHF values in singlehyphen;reference calculations, and to decrease these excitation energies relative to MCSCF values in multireference calculations, bringing the singlehyphen;reference and multireference CI 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 0ndash;0 transition energies are 1.9 and 2.9 eV, respectively.
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