Path integral molecular dynamics simulations for the H-6(+) and D-6(+) cluster cations have been carried out in order to understand the floppy nature of their molecular structure due to quantum-mechanical fluctuation. A full-dimensional analytical potential energy surface for the ground electronic state of H-6(+) has been developed on the basis of accurate ab initio electronic structure calculations at the CCSD(T)/cc-pVTZ level. It is found that the outer H-2(D-2) nuclei rotate almost freely and that the probability density distributions of the central H-2(D-2) nuclei show strong spatial delocalization. (c) 2007 Elsevier B.V. All rights reserved.