High-Level Ab Initio Electronic Structure Calculations of Water Clusters (H₂O)₁₆ and (H₂O)₁₇: A New Global Minimum for (H₂O)₁₆

Abstract

The lowest-energy structures of water clusters (H 2 O) 16 and (H 2 O) 17 were revisited at the MP2 and CCSD(T) levels of theory. A new global minimum structure for (H 2 O) 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 (H 2 O) 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 (H 2 O) 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][16][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) 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 small 30,31,35-37 and medium-sized 9-12,38,39 water clusters to the study of ...

Mapping the genome of meta-generalized gradient approximation density functionals: The search for B97M-V

“…In order to test the performance of these density functionals on the relative energetics of water clusters, the relative energies of 5 isomers of (H 2 O) 16 will be evaluated in the aTZ basis set with the (99,590)/SG-1 grid against reference values 139 computed at the CCSD(T)/aTZ level of theory.…”

Communication: Energy benchmarking with quantum Monte Carlo for water nano-droplets and bulk liquid water

“…Figure 1 shows the errors (deviations from DMC benchmarks) of BLYP-1 and BLYP-2 for samples of 100 configurations of the (H 2 O) 6 and (H 2 O) 15 nanodroplets. (Further details of the configuration samples and the DFT calculations, and results for (H 2 O) 9 are in the supplementary material. 15 ) The errors are plotted against the squared radius of gyration R 2 gyr of the cluster, defined as R 2 gyr = N −1 n |r n −r| 2 , with N the number of monomers, r n the O position of monomer n, andr = N −1 n r n the centroid of O positions.…”

Communication: Rigorous calculation of dissociation energies (D) of the water trimer, (H2O)3 and (D2O)3

“…The harmonic approximation, which is often used to estimate ZPEs, is suspect for water clusters, which are fluxional and which contain numerous low-lying structural isomers. [1][2][3][4][5][6][7][8][9][10] Thus, a rigorous calculation of the ZPE must take account of the vibrational anharmonicity, which, given the existence of low-lying isomers mentioned above, may be quite extreme and thus computationally highly demanding. Even for the smallest cluster, the water dimer, which has twelve vibrational degrees of freedom, this calculation is very demanding.…”