Accurate And Rapid Evaluation Of The Many-body Contributions To The Intermolecular Interactions In Water Clusters

In the nature,water molecules are usually linked by hydrogen bonds and other intermolecular interactions to form a giant network structure.The many-body expansion method can be used to decompose the total energy of water clusters into the energy algebraic summation over multiple sub-systems so that the properties of the clusters can be obtained quickly.Therefore,accurate and rapid evaluation of the interaction energies for water molecules plays a vital role in understanding the properties and functions of water clusters.CCSDT/CBS method is used as a high-accurately theoretical level but it is limited to apply on the study of middle-size water clusters due to the extensive calculation.It is very necessary to propose a method which can be used to calculate the energies efficiently and accurately.In this dissertation,(H₂O)₃ and(H₂O)₂ water clusters were studied by using CCSDT method at the complete basis set limit(CBS,extrapolated by using CCSDT results evaluated at cc-p VQZ and cc-p V5Z and also the HF result evaluated at cc-p V6Z).The computation efficiency and accuracy of related ab initio or density functional theory methods were also tested and discussed.The main conclusions obtained from this dissertation are as below:1.At the complete basis set limit,the accuracy of MP2 or CCSD(T)method is good but that of CCSD method is poor.The accuracy of the many-body interaction energies evaluated at MP2/CBS level(extrapolated according to the results obtained by using cc-p VTZ and cc-p VQZ basis sets after considering the basis set superposition effect)is relatively high.The absolute deviation is less than 0.1 kcal/mol.2.The accuracy of the interaction energies evaluated by adding half of the basis set superposition correction energy to the MP2/cc-p VQZ level is relatively high.The absolute deviations of the two-body interaction energies obtained by this method are less than 0.6kcal/mol,and the relative deviations of that are less than 5%.3.Dozens of density functionals theory methods combined with Pople or Dunning basis sets have also been tested to calculate many-body interaction energies.The results show that the?B97XD/aug-cc-p VTZ level is the most accurate in comparision with other methods.4.The many-body interaction energies of(H₂O)₆ and(H₂O)₇ water clusters are also calculated at the?B97XD/aug-cc-p VTZ level.When the two-body effect which the shortest intermolecular O···H distances is less than 0.25 nm is considered,more than half of the computation time is saved and the relative deviation is less than 15%.When the two-body effect which the shortest intermolecular O···H distances is less than 0.40 nm is considered,about 6%of the computation time is saved and the relative deviation is less than 2%.It is worth mentioning that with the increase of the system,the reduction in the amount of calculation increase substantially,but the calculation error keeps basically unchanged.5.The relationship between the many-body interaction energies of the water clusters[(H₂O)_n(n=6,7)]and the total interaction energies was also discussed in this paper.It was found that the contribution of the two-body interaction energies to the total interaction energies is79%,and the contribution of the three-body interaction energies to the total interaction energies is about 20%.Four-body and the other higer order interactions are minor.These results are consistent with the previous findings.We hope that the results obtained in this dissertation could provide a theoretical evidence for the prediction of the structures and energies for water clusters rapidly and accurately.