A density functional study of CuO 2 molecules: structural stability, bonding and temperature eects Y. Pouillon, C. Massobrio * Institut de Physique et de Chimie des Mat eriaux de Strasbourg, 23 rue du Loess, F-67037 Strasbourg, France Received 7 September 2000; in ®nal form 3 October 2000 Abstract Structural and electronic properties of neutral and anionic CuO 2 molecules are investigated within density functional theory. The lowest energy structures are the bent CuOO and the linear OCuO  . Consideration of temperature eects via ®rst-principles molecular dynamics simulations allows to conclude that two CuO  2 isomers (bent CuOO  and CuO  2 side-on) coexist at very close energies in the measured photoelectron spectrum, for dierent spin states. Among the isomers of CuO 2 , bonding is the most covalent in the linear OCuO molecule. Ó 2000 Elsevier Science B.V. All rights reserved. The investigation of structural and bonding properties for small CuO clusters is a necessary prerequisite to the understanding of more complex copper oxide systems which contain CuO 2 units as building blocks [1]. In addition to their intrinsic fundamental interest, related to the evolution of the Cu±O interaction with increasing size, these studies are relevant for numerous technological applications, such as catalysis and corrosion, in- volving the reaction of oxygen with transition metal atoms. Matrix infrared adsorption [2,3] and electron spin resonance [4±6] in the gas phase have suggested the existence of the CuO 2 complex, which exhibits spectral features clearly distinct from those of the OCuO linear molecule. This latter forms in solid matrices via photoexcitation [2,3,7] and has also been observed in gas phase photoelectron spectroscopy from the anionic counterpart [1,8]. For the CuO 2 complex two ge- ometries have been proposed [8], the CuOO bent C s  and the CuO 2 side-on (C 2v ). However, for both neutral and negatively charged species, the identi®cation and assignment of spectral features to either one of these geometries is a matter of controversy, since no clearcut evidence could be collected favoring either one of the two isomers [8]. First-principles calculations have been per- formed at dierent levels of complexity mostly on the neutral species OCuO, CuO 2 side-on and CuOO. The linear form has been considered by CI investigations, but not in conjunction with the CuO 2 complexes [9,10]. For these two, separate CI calculations have predicted the C 2v structure to be 0.03±0.04 eV more stable than the C s one [11,12]. This contrasts with a study at the density func- tional theory (DFT) level employing basis sets of dierent sizes and the B3LYP functional for ex- change and correlation [13]. In Ref. [13] the CuOO isomer is found to be the most stable among the 1 December 2000 Chemical Physics Letters 331 (2000) 290±298 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +333-881-07249. E-mail address: Carlo.Massobrio@ipcms.u-strasbg.fr (C. Massobrio). 0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 1 9 3 - 3