Evaluation Of CBS Extrapolation Approaches And Novel Extrapolation Approaches Using NZaP Basis Set

At present,the quantum chemistry calculation is more and more widely used in chemistry and other related disciplines.The biggest problem for quantum chemistry researchers is how to choose the efficient calculation level and how to analyze and use the calculated data to get the correct conclusions.When we increase the size of basis set,the accuracy of calculation is improved gradually,but the computing resources are increasing.Due to the incompleteness of basis set,the calculation accuracy will be limited.So the complete basis set extrapolation can reduce the influence of the truncation error on the calculation result,and make it closer to the actual value.Therefore,the topic of this paper is different calculation methods and basis sets are selected to calculate and extrapolate.The main contents are as follows:(1)Comparison of different basis sets.In order to compare with nZaP basis sets and Dunning’s correlation-consistent basis sets,we use n ZaP basis sets and Dunning’s correlation-consistent basis sets to the quantum chemistry calculation,we calculate the SCF energies based on 22 molecules at HF/cc-pVnZ,HF/aug-cc-p Vn Z,HF/nZaP and HF/n ZaPa levels,respectively,then we analyse and compare the results,we make conclusion that the accuracy order of different basis sets is nZaPa>n ZaP>aug-cc-pVnZ>cc-pVnZ.(2)Hartree-Fock energy extrapolation.In order to obtain high precision results,we often use large basis sets,but the computing time is increasing.In order to shorten the calculation time,we use small basis set to extrapolate to the complete basis set,which not only save the computation time,but also get high accuracy.We make complete basis set extrapolation at HF/cc-pVnZ,HF/aug-cc-pVnZ and HF/nZaP levels,respectively.It can found that extrapolate the SCF energies at the HF/nZaP level to the CBS limit by formula(3.6)is the best method,and the CBS extrapolation improves the accuracy of the results.(3)MP2 energy extrapolation.The MP2 calculation method is used by many researchers,but the MP2 calculation is time-consuming,so the MP2 energy extrapolation is the key point of the research.We know that MP2 energy include SCF energy and MP2 electron correlation energy,the key point of MP2 energy extrapolation is the MP2 electronic correlation energy extrapolation.We extrapolate MP2 electron correlation energy at MP2/nZaP level to the CBS limit.It can be found that the CBS extrapolation greatly reduces the computation time and improves the computational accuracy.This paper focuse on the introduction,comparison and application of nZaP basis sets,and research on the Hartree-Fock energy and MP2 energy CBS extrapolation,then analyse and compare the extrapolation results,the reasonable extrapolation scheme will be produced.This is the great help for the quantum chemistry calculation.