The formation of new silicon cages: a semiempirical theoretical investigation Ju-Guang Han a,b, * , Zhao-Yu Ren c , Liu-Si Sheng a , Yun-Wu Zhang a , Jorge A. Morales d , Frank Hagelberg e a National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, People’s Republic of China b The Key Laboratory of Structural Biology of Chinese Academy of Sciences, Hefei 230029, People’s Republic of China c Institute of Photonics & Photon-Technology, Northwestern University, Xi’an 710046, People’s Republic of China d Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409, USA e Computational Center for Molecular Structure and Interactions, Department of Physics, ATM Science and General Science, Jackson State University, Jackson, MS 39217,USA Received 8 April 2002; revised 19 November 2002; accepted 9 December 2002 Abstract A systematic theoretical study on the Si n (n ¼ 26 – 36, 60) cages using semiempirical methods is herein presented. Equilibrium geometries, harmonic vibrational frequency analyses, enthalpies of formations, HOMO – LUMO energy gaps and other properties are calculated at the level of the AM1 theory. Present theoretical results show that all the Si n (n ¼ 26 – 36, 60) cage isomers exhibiting maximum symmetry undergo slight distortions into more stable structures of lower symmetry. No simple function for the clusters stability with respect to the number of Si atoms can be discerned in the whole Si n (n ¼ 26–36, 60) series. However, the stability in the Si n (n ¼ 33–36) series decreases as the number of Si atoms drops whereas the Si n (n ¼ 26–32) series shows the same trend with the exception of n ¼ 28 and 31. Several instances of four-fold coordinated, distorted sp 3 hybridized Si atoms carrying large net charges have been detected. Calculated HOMO – LUMO energy gaps of these Si n cages lie in the range of 3.9 – 4.6 eV, being larger than those of both smaller Si n clusters and bulk Si. Comparison with available theoretical and experimental data give firm support to the present application of semiempirical methods to large Si n clusters. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon fullerene cages; Geometry structure; Stability; HOMO–LUMO gap 1. Introduction There is currently a great interest in utilizing small atomic clusters as constituent elements in well- controlled nanostructures [1–17]. In the case of silicon clusters, this interest is obviously related to the semiconductor properties of silicon as illustrated 0166-1280/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0166-1280(02)00788-1 Journal of Molecular Structure (Theochem) 625 (2003) 47–58 www.elsevier.com/locate/theochem * Corresponding author. Address: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061, USA. Fax: þ 1-86-551-3603754. E-mail address: jghan@ustc.edu.cn (J.-G. Han), hjg212@ yahoo.com (J.-G. Han).