Journal of Electron Spectroscopy and Related Phenomena, 40 zyxwvutsrqponmlkjihgfedcbaZYXW (1986) 193-198 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Short communication zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE A GREEN’S FUNCTION CALCULATION OF THE LOWEST IONIZATION POTENTIAL OF SOME RADICALS P. TOMASELLO* and W. VON NIESSEN Znstitut fiir Physikalische und Theoretische Chemie, Technische W aivers&it Braunschweig, D-3300 Braunschweig (Federal Republic of Germany ) J. SCHIRMER and L.S. CEDERBAUM Znstitut fiir Physikalische Chemie, Universitiit Heidelberg, D-6900 Heidelberg (Federal Republic of Germany ) (Received 27 January 1986) ABSTRACT By using the so-called two particle-hole extended Tamm-Dancoff Approximation (2ph ex-TDA) to the one-particle Green’s function (GF), we calculated the lowest ionization potentials (IP) for the radicals NO, NS, PO, PS, CF, NH2, HCO and FCO. Though these molecules have an open-shell electronic structure, for which the usual (closed shell) GF development cannot be applied, we circumvented this difficulty by reversing in the actual calculation the physical order of the ionization process. The underlying ab initio SCF calculations were performed by using basis sets of double- to triple-zeta quality plus polarization functions. A comparison of the 2ph ex-TDA results with experiment shows the satisfactory usual accuracy (‘0.2 eV) of this method. A short comparison with other ASCF results is also done. A theoretical prediction for the ionization potential of the PS radical, where no experimental values are available, is finally proposed. INTRODUCTION The method of the Green’s function (OF) [l] has, in the last decade, become a very effective means of calculating the one-body electronic structure properties of atoms and molecules [2]. In particular, the * On leave from the Physics Department, Catania University, Italy. 0368-2048/86/$03.50 0 1986 Elsevier Science Publishers B.V.