Table 1 Lattice parameters and bond lengths of crystal structures optimized using GGA-PBE and vdW-DF methods (see Fig. 1(a) for the atomic labels)
 |  |  | Bond length (Å) | ||
---|---|---|---|---|---|
 | a (Å) | c (Å) | Bi1-I | I-Te1 | Te1-Bi2 |
GGA-PBE | 4.431 | 18.555 | 3.291 | 4.595 | 3.082 |
vdW-DF | 4.431 | 16.921 | 3.295 | 3.850 | 3.083 |
 | Bi2-Te2 | Te2-Bi3 | Bi3-Te3 | Te3-Te4 | Te4-Bi1 |
GGA-PBE | 3.292 | 3.278 | 3.087 | 4.249 | 3.075 |
vdW-DF | 3.280 | 3.254 | 3.094 | 3.746 | 3.076 |