Vol. 45 No. 1 SCIENCE IN CHINA (Series B) February 2002 Molecular structure of diatomic lanthanide compounds CAO Xiaoyan (曹晓燕) 1,3 , LIU Wenjian (刘文剑) 2 & Michael Dolg 1 1. Institut für Physikalische & Theoretische Chemie, Bonn 53115, Germany; 2. Lehrstuhl für Theoretische Chemie, Ruhr-Universität, Bochum 44780, Germany; 3. Biochemistry Department, Zhongshan University, Guangzhou 510275, China Correspondence should be addressed to Michael Dolg (email: dolg@thch.uni-bonn.de) Received July 24, 2001 Abstract The molecular constants of selected diatomic lanthanide compounds (LaH, LaO, LaF, EuH, EuO, EuF, EuS, GdO, GdF, GdH, YbH, YbO, YbF, YbS, LuH, LuO and LuF) have been cal- culated by using relativistic small-core pseudopotentials and optimized (14s13p10d8f6g)/ [6s6p5d4f3g] valence basis sets. The results are in good agreement with available experimental data, with exception of YbO and LuF. The reasons for the discrepancies in case of YbO are due to a complicated mixing of configurations in the ground state, whereas in case of LuF the binding energy estimated by experimentalists appears to be too low. Keywords: molecular structure, lanthanide element, pseudopotential, valence basis set. In the past three decades, the study of lanthanide compounds has received much attention. However, the complex electronic structure of the lanthanide elements poses a considerable chal- lenge for both experimental and theoretical work [1—5] . Both relativistic and correlation effects should be included in theoretical studies. Till now, relativistic ab initio pseudopotentials (PP) in combination with high-level wavefunction-based correlation treatments and relativistic density functional theory (DFT) methods are the two most effective methods for theoretical lanthanide chemistry. Because of the high electron correlation in the 4f shell and its weak but noticeable con- tribution to chemical bonding, 4f electrons should be treated as valence electrons in accurate ab initio calculations. The 4s, 4p, 4d orbitals and the 4f orbitals are in the same spatial region, and should be treated as valence electrons too in order to get a consistent description of electron inter- actions. That is the reason why a small-core PPs (28 core-electrons) are chosen [6] . Another prob- lem is the choice of appropriate valence basis sets. Because of the importance of g functions in describing 4f-shell correlation effects, the authors of this paper energy-optimized (14s13p10d8f6g)/ [6s6p5d4f3g] basis sets adopting a generalized contraction scheme based on atomic natural orbi- tals [7] . In this contribution we present molecular constants for selected diatomic lanthanide com- pounds derived with the small-core PPs and the optimized valence basis sets in correlated calcula- tions, and compare our results with available experimental data. 1 Methods Molecular structure constants were calculated at the CI (SD)+Q (single reference configura- tion interaction including single and double excitations and Davidson’s correction) level for GdH,