Chemical Physics I35 ( 1989) 357-361 North-Holland, Amsterdam CALCULATION OF THE VALENCE IONIZATION ENERGIES OF Ni(C0)4 AND CI(NO)~ Stephen SMITH, Ian H. HILLIER Department of Chemistry University ofManchester, Manchester Ml3 9PL, UK Wolfgang VON NIESSEN Institutfiir Physikalische und Theoretische Chemie, Technische Vniversitiit Braunschweig, O-3300 Brunswick, FRG and Martyn F. GUEST SERC Daresbury Laboratory, Warrington WA4 4AD, UK Received 2 1 February 1989 The valence ionization energies of Ni(CO), and Cr(N0)4 are calculated by a number of methods. The highest level of theory includes both electron relaxation and correlation in the ions, and electron correlation in the unionized molecule. The value of the various methods is assessed by comparison with the experimental photoelectron spectra. It is found that the extended two-parti- cle-hole Tamm-Dancoff method yields the best agreement with the experimental data. 1. Introduction LOW energy photoelectron spectroscopy has proved invaluable in the study of the electronic structure of transition metal complexes [ 11. However, the accu- rate calculation of the ionization energies (IEs) of such molecules is still a formidable theoretical chal- lenge. In the case of larger organic molecules, valence IEs may be predicted to within x 0.5 eV when both electron relaxation and correlation effects are in- cluded. This can be accomplished either by an SCF calculation followed by a configuration interaction (CI) expansion, or by a Green function calculation where both effects are considered and the whole manifold of IEs are readily obtained. However, in transition metal complexes the situation is somewhat more complicated. Here, the large differential relax- ation energy associated with ionization of metal- and ligand-localized electrons results in larger deviations from Koopmans’ theorem (KT) [ 11. Furthermore, in the first ab initio Green function calculation of the IEs of a transition metal complex we have shown that there may also be differential correlation effects as- sociated with the ionization of metal- and ligand-lo- calized electrons [ 21. To provide more insight into the value of the various methods of calculating the IEs of transition metal complexes, we here present calculations of the valence IEs of Ni( CO), and Cr(N0)4. The photoelectron spectrum of Ni( CO), has re- cently been reinvestigated by Reutt et al. [ 3 1, and the first nine IEs have been measured and assigned. The spectrum of Cr( NO), has been recorded by Plum- mer et al. [4], and five ionization bands have been identified. 2. Computational details The calculations were carried out at the experi- mental molecular geometries [ 51. The valence IEs were calculated at a number of levels of approximation: (i) From the SCF eigenvalues by the use of Koop- mans’ theorem. (ii) At the ASCF level, by carrying out an SCF cal- 0301-0104/89/$03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )