Electron Affinity of the O 2 Molecule: CCSD(T) Calculations Using the Optimized Virtual Orbitals Space Approach MARTIN S ˇ ULKA, MICHAL PITON ˇ A ´ K, PAVEL NEOGRA ´ DY, MIROSLAV URBAN Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska ´ dolina, SK-842 15 Bratislava, Slovakia Received 12 March 2008; accepted 19 March 2008 Published online 20 May 2008 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.21743 ABSTRACT: The electron affinity (EA) of the oxygen molecule is calculated by the CCSD(T) method using the optimized virtual orbitals space (OVOS) technique by which the dimension of the original space of virtual orbitals can be significantly reduced. Extended basis sets, up to the doubly augmented correlation consistent d-aug-cc-pV6Z basis sets for O 2 and O 2 - at their experimental geometries are used. We demonstrate that even when the space of virtual orbitals is reduced to 50% of the full space, the resulting EA of the O 2 molecule is accurate to within 0.01 eV. At the same time, the computational effort is reduced by about an order of magnitude. With OVOS reduced to 60% of the full virtual space, results are almost accurate. Considering the complete basis set limit with so reduced OVOS, corrections for the core correlation, vibrational and relativistic effects, the electron affinity of O 2 is 0.452  0.01 eV. This value agrees very well with the full virtual orbital space, EA = 0.446 eV, and with the recent experimental value, EA = 0.448  0.006 eV. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem 108: 2159 –2171, 2008 Key words: electron affinity; O 2 ; CCSD(T); OVOS Correspondence to: M. Urban; e-mail: urban@fns.uniba.sk Contract grant sponsor: Slovak Research and Development Agency. Contract grant number: APVV-20 – 018405. Contract grant sponsor: VEGA. Contract grant number: 1/3560/06. This paper is dedicated to Professor Karol Jankowski at the occasion of his 70th birthday as the appreciation of his outstand- ing contribution to the developments in the field of quantum chemistry. International Journal of Quantum Chemistry, Vol 108, 2159 –2171 (2008) © 2008 Wiley Periodicals, Inc.