Application of accelerated-convergence technique to modified Lanczos calculations

Summary The use of modified Lanczos procedure to obtain the ground-state eigenvalue and eigenvector of the Hamiltonians of physical systems and its subsequent modification for the determination of excited states has been revealed a very efficient and powerful procedure. Nevertheless, when implementing the method, one notices that the convergence of the algorithm is very slow, so that the calculation may become heavy from a computational point of view. The aim of this paper is the proposal of an accelerated-convergence technique (which has already been successfully used in density-of-states calculations), that allows to obtain a faster convergence of the modified Lanczos eigenvalues to the exact ones. The advantages of such a technique are shown through the discussion of some examples.