Density Functional Theory (DFT) studies on the ground state of N03(2A'2) radical and the first triplet state of NO; cation ~~ ~ CAO, Xiao-Yan"($?&%) LI, Le-Minb(%%R) XU, G ~X i a n * ( ~% % ) "State Key Laboratory for Structurd Chemistry of Unstable and Stuble S p c k , Instit& of Chmktry, Chime Academy of 'State Key Laboraimy of Rare Earth Materials Chemjstv and Applicatwns , College cf Chemistry and Molecular Engineer-HONG, Gong-Yi" ( 8 ?d X ) WANG , Dian-Xun". ( € E? fd~ ) Scimces, Beying 100080, China ing, Peking University, Beijing 1W71, China Intraduction The determination of the p u n d electronic state and molecular structure of the nitrate radical ( N Q ) has been of great interests for both experimentalists and theorists. '-'O A challenging question existing is to determine that the p u n d state of N& possesses D3h or czv symmetry. It has been investigated by using various ab initw method, such as MPlT,4 CASSCF Couple-Cluster method" as well as MR-CI.4*9 However, there are different answers to this question based on Merent levels of theory. For example, while MP2 and Ml ' 4 predict D3h symmetry, MP3 favors CZv symmetry.4 The Couple-Cluster calculations both on Bmeckner determi-