Convergence analysis of a quasi-continuum approximation for a two-dimensional material without defects

In many applications, materials are modeled by a large number of particles (or atoms), where any particle can interact with any other. The computational cost is very high since the number of atoms is huge. Recently much attention has been paid to a so-called quasi-continuum (QC) method, which is a mixed atomistic/continuum model. The QC method uses an adaptive finite element framework to effectively integrate the majority of the atomistic degrees of freedom in regions where there is no serious defect. However, numerical analysis of this method is still in its infancy. In this paper we will conduct a convergence analysis of the QC method in the case when there is no defect. We will also remark on the case when the defect region is small. The difference between our analysis and conventional analysis is that our exact atomistic solution is not a solution of a continuous partial differential equation, but a discrete lattice scale solution which is not approximately related to any conventional partial differential equation.