Water cluster basis sets

As it is now very well known, accurate studies of the water-water interaction by means of ab-initio techniques require the use of larger and flexible basis sets and methods which consider correlation effects [85,94-96], Since high level ab-initio post-Hartree-Fock calculations are unfeasible because of their high computational cost for systems with many degrees of freedom, Density Functional Theory, more economical from the computational point of view, is being more and more considered as a viable alternative. Recently, we have presented [97] results of structural parameters and vibrational frequencies for the water clusters (H20) , n=2 to 8, using the DFT method with gradient corrected density functionals. 

Various energy terms calculated for the linear water dimer molecule and for some hexamer water clusters at the HF level in both the canonical and the separated representations, using various basis sets are given in the present work. This study is twofold. First, the differences between the proton donor and proton acceptor molecules have been assessed in the linear water dimer. The EM energy contributions (obtained in the separated representation) are expected to characterize the proton donor / acceptor ability of the monomers. Secondly, a possibility of identification of a proton donor or acceptor character of the monomers in some water hexamers was tested. 

TABLE IV. Energy terms in two basis sets for some water hexamer clusters (separated representation). Values are given in hartree. 

Figure 4.12 n —s> 7t vertical excitation energy in formamide-water clusters with various stand-alone methods. L, M, and S stand for the large (6-311+C(d,p)), medium (6-31C(d))r and small (3-21C) basis sets, respectively. 

In general, for each acid HA, the HA-(H20) -Wm model reaction system (MRS) comprises a HA (H20) core reaction system (CRS), described quantum chemically, embedded in a cluster of Wm classical, polarizable waters of fixed internal structure (effective fragment potentials, EFPs) [27]. The CRS is treated at the Hartree-Fock (HF) level of theory, with the SBK [28] effective core potential basis set complemented by appropriate polarization and diffused functions. The W-waters not only provide solvation at a low computational cost they also prevent the unwanted collapse of the CRS towards structures typical of small gas phase clusters by enforcing natural constraints representative of the H-bonded network of a surface environment. In particular, the structure of the Wm cluster equilibrates to the CRS structure along the whole reaction path, without any constraints on its shape other than those resulting from the fixed internal structure of the W-waters. 

Wang 2 determined that the glycine zwitterion water cluster is an artifact of method and basis set HF yields a local energy minimum, as does B3LYP and MP2 so long as the basis set does not include polarization functions on... 

Figure 7.2 Quasi-chemical contributions of the hydration free energy of Be (aq). Cluster geometries were optimized using the B3LYP hybrid density functional (Becke, 1993) and the 6-31- -G(d, p) basis set. Frequency calculations confirmed a true minimum, and the zero point energies were computed at the same level of theory. Single-point energies were calculated using the 6-311- -G(2d, p) basis set. A purely inner-shell n = 5 cluster was not found the optimization gave structures with four (4) inner-sphere water molecules and one (1) outer-sphere water molecule. For n = 6 both a purely inner-shell configuration, and a structure with four (4) inner-shell and two (2) outer-shell water molecules were obtained. The quasi-chemical theory here utilizes only the inner-shell structure. O - rin [/ff -f (left ordinate) vs. n. A ...

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