An accelerated polynomial expansion scheme to construct the density matrix in quantum mechanical molecular dynamics simulations is proposed. The scheme is based on recursive density matrix expansions, e.g., [A. M. N. Niklasson, Phys. Rev. B, 66 (2002), 155115], which are accelerated by a scale-and-fold technique [E. H. Rubensson, J. Chem. Theory Comput., 7 (2011), pp. 1233-1236. The acceleration scheme requires interior eigenvalue estimates, which may be expensive and cumbersome to come by. Here we show how such eigenvalue estimates can be extracted from the recursive expansion by a simple and robust procedure at a negligible computational cost. Our method is illustrated with density functional tight-binding Born-Oppenheimer molecular dynamics simulations, where the computational effort is dominated by the density matrix construction. In our analysis we identify two different phases of the recursive polynomial expansion, the conditioning and purification phases, and we show that the acceleration represents an improvement of the conditioning phase, which typically gives a significant reduction of the computational cost.
Introduction.With the fast growth of computational processing power, atomistic simulations based on calculations of the electronic structure have become a powerful approach to the study of a broad range of problems in materials science, chemistry, and biology [26,33,64]. Nevertheless, the computational cost associated with electronic structure calculations normally limits applications to fairly small systems. In particular, the cubic, O(N 3 ), scaling of the computational cost as a function of the number of atoms, N , for the regular solution of the quantum mechanical eigenvalue problem is considered to be a most limiting factor. A number of different electronic structure technologies have therefore been developed that circumvent this bottleneck with a computational effort that scales only linearly with the system size [6,18]. The reduction in the cost is typically achieved by utilizing sparse matrix algebra in an iterative construction of the density matrix, which avoids the full regular solution of the quantum mechanical eigenvalue problem. The matrix sparsity arises in localized atomic basis set representations due to the short-range character of the electronic wavefunctions for nonmetallic materials [3,27,28]. With linear scaling