pubs.acs.org/JPCL
High-Level Ab Initio Electronic Structure Calculations of Water Clusters (H2O)16 and (H2O)17: A New Global Minimum for (H2O)16 Soohaeng Yoo,† Edoardo Apr a,‡ Xiao Cheng Zeng,*,§ and Sotiris S. Xantheas*,† †
Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, MS K1-83, Richland, Washington 99352, United States, ‡Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States , and §Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States
ABSTRACT The lowest-energy structures of water clusters (H2O)16 and (H2O)17 were revisited at the MP2 and CCSD(T) levels of theory. A new global minimum structure for (H2O)16 was found at both the MP2 and CCSD(T) levels of theory, and the effect of zero-point energy corrections on the relative stability of the low-lying minimum energy structures was assessed. For (H2O)17, the CCSD(T) calculations confirm the previously found at the MP2 level of theory “interior” arrangement (fully coordinated water molecule inside a spherical cluster) as the global minimum. SECTION Molecular Structure, Quantum Chemistry, General Theory
O
ne of the major challenges in the theoretical/computational community currently lies with the development of classical water potential models that accurately account for underlying intermolecular nonbonding interactions, such as hydrogen bonding and dispersion interactions.1-32 Algorithmic developments targeted toward obtaining low-lying local minima and the global minimum of a complex, multidimensional potential energy surface (PES), have received a lot of attention when applied to medium size water clusters (H2O)n (3 e n e 21).1-14,33 These systems constitute an important test-bed for benchmarking new global sampling approaches, while at the same time provide useful information about the relative stability of entirely different hydrogen bonding arrangements, such as the ones found at interfaces, in confined water, and at ordered aqueous phases. The accurate account of the subtle energy differences between the low-lying minima of water clusters, especially those corresponding to dissimilar hydrogen bonding networks resulting from the need to maximize hydrogen bonding in finite systems, are essential in both the parametrization and the assessment of the accuracy of classical transferable force fields for water.15-17,27,29,34 These transferable models are intended to be used in different aqueous environments such as water clusters, liquid water, ice, at aqueous interfaces, and around charged species or hydrophobic surfaces; the more accurately they account for the subtle energetic differences between the different networks, the more reliable they can be expected to be in describing these diverse hydrogen bonding environments. Given the fact that there is currently no direct experimental measurement of the binding energy of even the simplest water cluster, viz. the water dimer, first principles (ab initio)
r 2010 American Chemical Society
electronic structure calculations have been previously used in order to obtain accurate estimates of the binding energies of water clusters at increasingly higher levels of electron correlation and with basis sets that approach the complete basis set (CBS) limit. These results constitute an indispensable and currently irreplaceable path toward understanding aqueous environments, via the assessment of the accuracy of classical and quantum force fields for water. In this study we extend our previous studies of small30,31,35-37 and medium-sized9-12,38,39 water clusters to the study of the low-lying networks of the (H2O)16 and (H2O)17 clusters. These previous studies have been performed at the second order Møller-Plesset perturbation40 level of theory (MP2). It should be noted that, to date, the assessment of the effect of higher order electron correlation at the more accurate (but also highly expensive) CCSD(T) level of theory has been reported for water clusters only up to n = 6 molecules.32 We have recently shown the possibility of obtaining CCSD(T)/ aug-cc-pVTZ energies for water clusters up to n = 24 molecules using highly scalable electronic structure software on parallel computer architectures.41 We have used this approach to assess the effect of higher electron correlation at the CCSD(T) level for the relative stability of the n = 16 and 17 clusters and compare them with the corresponding results obtained at the MP2 level. To this end, these results represent the current state-of-the-art in scalable high-level electronic structure theory. They also provide the necessary information needed to assess the accuracy of schemes used
Received Date: September 2, 2010 Accepted Date: October 3, 2010 Published on Web Date: October 12, 2010
3122
DOI: 10.1021/jz101245s |J. Phys. Chem. Lett. 2010, 1, 3122–3127
pubs.acs.org/JPCL
the HF energy) and 0.00045 au/bohr (convergence of the gradient) were used during the geometry optimizations. No basis functions were disregarded because of linear dependencies when building the molecular orbitals. Single point Coupled Cluster Singles and Doubles with a perturbative estimate of the Triples excitations [CCSD(T)]55,56 with the aug-cc-pVDZ and aug-cc-pVTZ basis sets at the corresponding optimal MP2 geometries with those two basis sets were carried out for the first few most stable isomers. The CCSD convergence criterion was
