The equilibrium geometry and vibrational modes of H2S and H2O-terminated Ge( 001)-(2 x 1) surfaces are calculated in a supercell approach using first-principles density functional theory in the local density (LDA), generalized gradient ( GGA) approximations and van der Waals (vdW) interactions. Mode frequencies are found using the frozen phonon method. For the H2S-passivated surface, the calculated frequencies in LDA (GGA) are 2429 cm(-1) (2490) for the H-S stretch mode, 712 cm(-1) (706) for the H-S bond bending mode, 377 cm(-1) (36) for the Ge-S stretch mode and 328 cm(-1) (337) for H-S wag mode. Frequencies for the H2O passivated surface are 3590 cm(-1) (3600) for the H-O stretch mode, 921 cm(-1) (947) for the bending mode, 609 cm(-1) (559) for the Ge-O stretch, 1995 cm(-1) (1991) for the Ge-H stretch mode, 498 cm(-1) (478) for the Ge-H bending mode and 342 cm(-1) (336) for the H-O wag mode. The differences between the functionals including vdW terms and the LDA or GGA are less than the differences between LDA and GGA for the vibrational mode frequencies. 2014 Elsevier B.V. All rights reserved.