We present a theoretical study of the geometrical and electronic properties of the C- and Si-terminated beta-SiC(001) surfaces in the vicinity of the missing dimer defect. The experimental results suggest that the atomic structures of these two surfaces may be considerably modified by external stress. In our present study we have considered the possible influence of this factor on the surface geometry of both systems. We have shown that the structural differences between the C-and Si-terminated surfaces lead to their different behaviour in the presence of a missing dimer and applied stress. In the case of the C-terminated c(2 x 2) surface, the missing dimer defect causes the buckling of the adjacent carbon dimers lying in the line of the defect (dimer atoms adjacent to the defect have vertical positions lower by 0.18 angstrom). This effect becomes more pronounced in the presence of compressive stress - the stress of 8% leads to the buckling of these two dimers of around 0.5 angstrom. The vertical positions of silicon atoms located directly below the defect were increased by 0.2 angstrom. We have also found that the missing dimer influences the structure of the carbon dimers on the neighbouring lines of dimers. Contrary to the C- terminated surface, the missing dimer defect on the Si-terminated SiC(001)-p(2 x 1) surface remains neutral for silicon dimers located in the line of defect, i.e. the dimers do not change their geometrical properties in unstrained structure nor in the presence of a tensile stress. On the other hand, this defect modifies considerably the geometry of the dimers from the two neighbouring lines of dimers by reducing their bond lengths and vertical positions. Changes in the geometrical properties of the second neighbour dimers (with respect to the defect) in these two lines are also noticeable. Moreover, we have found that the presence of a missing dimer modifies significantly the positions of the adjacent subsurface carbon atoms. (C) 2008 Elsevier B. V. All rights reserved.