Accurate Prediction Of The Two-body Interaction Strength For Water Clusters

Biological systems usually contain large amounts of water molecules,and thus have a large number of many-body interactions.A deep understanding of the intermolecular interactions between water molecules has great value in the development of high precision molecular modeling methods.It has been shown that the two-body interaction in the water cluster can be the largest proportion of the total interaction,and the contribution of the three-body,four-body and higher-order interactions is small.Therefore,the accurate prediction of two-body interaction energy is very important.The results show that the CCSD?T?/CBS method can obtain very accurate two-body interaction energy,but for large water clusters,they always have the large number of two-body interaction,thus it is cost lost.So it is important that find a method with accurately and quickly to two-body interaction energy.In this thesis,the MP2/aDZ and MP2/aTZ methods containing the basis set superposition error?BSSE?correction,and the MP2/CBS,MP2.5/CBS,CCSD?T?/CBS methods were used to calculate the two-body interaction energies for water clusters?H₂O?_n?n=2-7,10,12,14,16,18,20?.The relationship between the two-body interaction strength and the intermolecular distance(the distance between the two oxygen atoms R_OO)was further examined.A combined method with efficiency and high accuracy was proposed.This thesis shows that the MP2.5/CBS method is as accurate as the CCSD?T?/CBS method in predicting the two-body interaction strength and this is validated for the smaller water clusters?H₂O?_n?n=2-7?.Compared to the MP2.5/CBS results,the MP2/aDZ,MP2/aTZ,MP2/CBS methods can produce two-body interaction energies accurate enough for those long distance(R_OO>0.41nm);however,for those short distance two-body interactions(R_OO?0.41nm),MP2/CBS outperforms MP2/aDZ and MP2/aTZ obviously.Therefore,a combine method is proposed in this thesis,that is,the MP2/aDZ method is used to calculate the long distance two-body interaction and the MP2/CBS method is used for the short distance two-body interaction.Calculation results manifest that compared to the MP2.5/CBS results,the combined method proposed in this thesis produces the total two-body interaction energies for 32 water clusters with a relative average error of only 1.1%and with a maximum relative error of only 1.5%,proving the combined method's utility.It is further found that the total two-body interaction energies obtained with the combined method,plus the total three-body interaction energies obtained at the MP2/aDZ level can be very close to the MP2.5/CBS total interaction energies,with the absolute error of only 1.43 kJ/mol and the relative error of only 0.6%.These further demonstrate that the combined method proposed in the thesis is useful.