Hartree-Fock and post-Hartree-Fock (density functional DFT, many-body perturbation theory MP2 and MP4(SDTQ), and coupled-cluster CCSD, and CCSD[T]) quantum-mechanical methods were applied to establish molecular structures and interaction energies of the GeH2 ... OH2 complex. Molecular geometries of nonplanar and planar conformers were optimized at the Hartree-Fock, DFT, and MP2 levels of theory using triple-zeta quality (TZP) augmented by sets of polarization functions (TZP(2d,2p) and TZP(2df,2pd)) basis sets. The optimized nonplanar structures correspond to minimum-energy species at all applied levels of theory. Calculated interaction energies (corrected for the basis set superposition error, CP) are relatively large and amount to -9.43 kcal/mol (MP4(CP)(SDTQ)/TZP(2df,2pd)//MP2/TZP(2df,2pd)), -9.77 kcal/mol (DFT(CP)/TZP(2df,2pd)//DFT/TZP(2df,2pd)), -8.16 kcal/mol (CCSD(CP)/TZP(2df,2pd)//MP2/TZP(2df,2pd) and -8.90 kcal/mol at the CCSD[T](CP)/TZP(2df,2pd) levels. The optimized planar molecular structure of the studied complex (MP2/TZP(2df,2pd) level) corresponds to the transition-state form (one imaginary, 66i cm(-1) harmonic vibrational frequency). A repulsive (2.07 kcal/mol) but minimum-energy planar structure of cm GeH2 OH2 is predicted at the DFT/TZP(2df,2pd) level of approximation. However, single-point calculations at the planar DFT-optimized geometry reveal this complex to be slightly nonbounded (0.01 kcaymol, CCSD(CP)) or very weakly bounded (-0.17 kcal/mol, CCSD[T](CP)).