Ab initio study of the ground state and the first excited state of the rectangular (D-2h) N-4 molecule

The dissociation pathway of D-2h N-4 has been investigated at the CASSCF le vel of theory. A new C-2v transition state to dissociation was found and ch aracterized on the potential energy surface. The effective barrier of disso ciation from D-2h N-4 to 2N(2) is estimated to 6.5 kcal/mol from MR-AQCCNTZ //CAS(12,12)/VTZ calculations. Vertical excitation energies and oscillator strengths for the lowest 20 singlet states of D-2h N-4 have been calculated using the EOM-CCSD method. The geometry of the first excited state B-1(3u) , considered as the lowest optically accessible state, has been optimized a nd characterized at various computational levels. This state might be usefu l for detection of D-2h N-4 by means of laser-induced fluorescence (LIF) sp ectroscopy. (C) 2001 Elsevier Science B.V. All rights reserved.