Abstract
A second order accurate (in time) numerical scheme is analyzed for the slope-selection (SS) equation of the epitaxial thin film growth model, with Fourier pseudo-spectral discretization in space. To make the numerical scheme linear while preserving the nonlinear energy stability, we make use of the scalar auxiliary variable (SAV) approach, in which a modified Crank-Nicolson is applied for the surface diffusion part. The energy stability could be derived a modified form, in comparison with the standard Crank-Nicolson approximation to the surface diffusion term. Such an energy stability leads to an H-2 bound for the numerical solution. In addition, this H-2 bound is not sufficient for the optimal rate convergence analysis, and we establish a uniform-in-time H-3 bound for the numerical solution, based on the higher order Sobolev norm estimate, combined with repeated applications of discrete Holder inequality and nonlinear embeddings in the Fourier pseudo-spectral space. This discrete H-3 bound for the numerical solution enables us to derive the optimal rate error estimate for this alternate SAV method. A few numerical experiments are also presented, which confirm the efficiency and accuracy of the proposed scheme.