Ab-initio and density functional theory calculations are used to study the geometrical details and vibrational properties of low-dimensional units that constitute the structure of elemental sulfur and arsenic sulfide glasses. Bond lengths, bond angles ands torsion angles have been estimated for the S-8 ring, the S-n helical chain and the cagelike units As4S4, As4S6 and As4S10. The vibrational frequencies of the two larger cages have been calculated and compared to experimental data.