Direct ab initio molecular dynamics calculations have been carried out for the H-6(+) cluster and its isotopic analogues: [H-2(H-2)H-2](+), [H-2(H-2)D-2](+), and [H-2(HD)H-2](+). The calculations predict that the average geometry over the trajectories is asymmetric in both the deuterated clusters [H-2(H-2)D-2](+) and [H-2(HD)H-2](+), causing disproportion in the spin density determining hyper. ne coupling constants (HFCCs). This prediction agrees well with our recent ESR study of the isotope effects on the HFCCs for these clusters that are considered present in gamma-irradiated solid p-H-2. The present result thus provides supportive theoretical evidence for the H-6(+) cluster formation in the solid p-H-2. (C) 2008 Elsevier B.V. All rights reserved.