Density functional calculations of the energy surfaces of infinite helical chains of selenium have been performed for a wide range of geometries using the parameter-free local density approximation. The results are compared with earlier calculations for the sulfur helix Sinfinity for which the calculated ground state geometry is in good agreement with experiment. The energy surfaces of both molecules are flat over large regions of configuration space-consistent with the extreme flexibility of these molecules-and are characterized by a variety of local minima and saddle points. Stretching each molecule results in planar zig-zag and linear geometries with overlapping conduction bands. The energy differences between helical and planar minima are small ( approximately 0.2 eV).
 

Journal of Chemical Physics, 88 2652-8, 1988.