Low-Lying Structures and Stabilities of Large Water Clusters: Investigation Based on the Combination of the AMOEBA Potential and Generalized Energy-Based Fragmentation Approach Zhen Yang, Shugui Hua, Weijie Hua, and Shuhua Li* School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of the Ministry of Education, Institute of Theoretical and Computational Chemistry, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: April 28, 2010; ReVised Manuscript ReceiVed: July 15, 2010 Based on a large database of local minima obtained with the polarizable AMOEBA potential, the generalized energy-based fragmentation (GEBF) approach is applied to locate low-lying structures of water clusters (H 2 O) n in the range n ) 20-30, at the B3LYP and MP2 levels. Our results show that the relative stabilities of isomers predicted by the AMOEBA empirical potential differ noticeably from those predicted by GEBF- B3LYP/6-311++G(d,p) and GEBF-MP2/6-311++G(3df,2p) calculations. From GEBF-B3LYP energies with zero-point vibrational energy corrections, one can see that for water clusters in the range n ) 20-30 the transition from one-centered to two-centered cage structure occurs at n ) 26. With increasing cluster size, the number of H-bonds per water molecule in the lowest-energy structures shows a gradually increasing trend, and the proportion of four-coordinated water molecules gradually increases, as expected for large water clusters. Based on GEBF-MP2/6-311++G(3df,2p) energies (instead of GEBF-B3LYP/6-311++G(d,p) energies), different lowest-energy structures can be found for six cluster sizes in the range n ) 20-30, suggesting the significance of the dispersion interaction in determining the relative energies of low-lying water clusters. 1. Introduction Understanding and predicting the properties of water clusters are of fundamental importance in many physical, chemical, and biological fields. An important purpose for such studies is to obtain a molecular-level description of the properties of the bulk water through the study of various water clusters, (H 2 O) n . 1-3 Recently, a large number of experimental 3-14 and theoretical studies 15-43 have been conducted to investigate the structures and properties of water clusters. Since the number of local minimum structures for (H 2 O) n increases exponentially with the size of water clusters, many effective optimization approaches have been developed to search the global minima of water clusters with different sizes by using empirical interaction potentials. 15-25 For example, Niesse and Mayne 15 used a genetic algorithm with the TIP3P 44 potential to explore the global minima of water clusters containing up to 13 molecules. Wales and co-workers 18,19 employed a basin-hopping (BH) method to find the global minima for n e 21 with the TIP4P 44 and TIP5P 45 potentials. Kazachenko and Thakkar 25 developed an improved minima-hopping algorithm to locate the global minima for n e 37. On the other hand, one can notice that different empirical potentials usually lead to different global minima, especially for relatively large water clusters (n > 10). For example, the global minima predicted by the TTM2-F potential agree qualitatively with those predicted by the TIP4P potential for small clusters n ) 2-11, but the results for larger clusters are not consistent with each other. 21 The global minimum structures for n ) 11-21 predicted with TIP4P and TIP5P potentials are quite different. 19 Due to different accuracies of various empirical potentials, global minimum structures predicted from various model potentials are better considered as good candidates for true global minima. Thus, it is still necessary to reoptimize the structures of these low-energy candidates using high-level ab initio calculations. Presently, a number of ab initio calculations have been performed for the low-lying structures of water clusters. 26-43 For example, Maheshwary et al. 32 have extensively investigated structures and stabilities of water clusters with n ) 8-20 at the Hartree-Fock (HF) levels with up to the 6-311++G(2d,2p) basis set. Fanourgakis et al. 33 reported more accurate energies for four low-lying families of (H 2 O) 20 at the second-order Møller-Plesset perturbation (MP2) level of theory. Lenz and Ojamae 35 performed geometry optimizations for a variety of water clusters with 8, 10, and 12 molecules, as well as for some larger clusters with up to 22 molecules, whose structures were constructed based on the information from small clusters. Up to now, the lowest-energy structures for n e 10 have been well established from various ab initio calculations, which are consistent with experimental studies. 26-30,39-43 How- ever, as mentioned above, ab initio calculations on larger water clusters (n g 20) have been limited to a relatively small number of low-energy structures. 32-37 It should be emphasized that an exhaustive search for the global minimum structures of large water clusters is not yet practical for current ab initio calcula- tions. Evidently, a feasible way of determining low-energy structures of water clusters is to first identify a large number of low-energy candidate structures with empirical potentials, and to further optimize the structures of these candidates using various ab initio methods. It is well-known that the computational cost of traditional algorithms for ab initio calculations increases steeply with the * To whom correspondence should be addressed. E-mail: shuhua@ nju.edu.cn. J. Phys. Chem. A 2010, 114, 9253–9261 9253 10.1021/jp1038267  2010 American Chemical Society Published on Web 07/29/2010