The thermal equilibrium state of H-5(+) is investigated by means of an ab initio path integral molecular dynamics (PIMD) method, in which degrees of freedom of both nuclei and electrons at finite temperature are quantized within the adiabatic approximation. The second-order Moller-Plesset force field has been employed for the present ab initio PIMD. At 5-200 K, H-5(+) is shown to have the structure that the proton is surrounded by the two H-2 units without any exchange of an atom between the central proton and the H-2 unit. At 5 K, the quantum tunneling of the central proton occurs more easily when the distance between the two H-2 units is shortened. At the high temperature of 200 K, the central proton is more delocalized in space between the two H-2 units, with less correlation with the stretching of the distance between the two H-2 units. As for the rotation of the H-2 units around the C-2 axis of H-5(+), the dihedral angle distribution is homogeneous at all temperatures, suggesting that the two H-2 units freely rotate around the C-2 axis, while this quantum effect on the rotation of the H-2 units becomes more weakened with increasing temperature. The influence of the structural fluctuation of H-5(+) on molecular orbital energies has been examined to conclude that the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap is largely reduced with the increase of temperature because of the spatial expansion of the whole cluster. (C) 2004 American Institute of Physics.