The algorithm for optimizing molecular geometries in redundant internal coordinates is extended to periodic systems. The lattice vectors are not explicitly included in the set of optimized coordinates, but are adjusted only implicitly via a combination of chemically meaningful inter- and intracell internal coordinates. The Wilson B matrix required for coordinate transformations is modified to include internal coordinate derivatives with respect to lattice vectors. The efficiency of the algorithm is demonstrated on a one-dimensional polymer, poly(p-phenylenevinylene), and a three-dimensional crystal of urea. (C) 2001 American Institute of Physics.