To assign infrared (IR) absorption bands related to Si-H vibrations in porous silicon, harmonic frequencies for model clusters were calculated by an ab initio molecular orbital (MO) method using a 6-31 G* basic set. Vibrations associated with a group of interest can be obtained by terminating the cluster with pseudoatoms of a heavy hydrogen isotope in the calculations. The calculated results reproduce the measured IR spectrum accurately, indicating that the isolation technique is effective, although the localization for some deformation modes, particularly the rocking modes, is insufficient at lower frequencies. It is concluded from the results of the calculated and measured spectra that there are clustered states such as (SiH2)(n) and (SiH)(2) in addition to the monomer states, SiHx, and that absorption bands at 600 to 700 cm(-1) are due to the vibrations of the SiHx bending or deformation mode rather than the Si-Si lattice vibration.