An ab initio path integral molecular dynamics simulation has been performed to study the quantum and thermal effects of a dihydrogen bonded cation, NH4+...CBeH2. In this system, an attractive interaction exists between two neighboring hydrogen atoms as Ndelta-Hdelta+... Hdelta-Bedelta+ involving large-amplitude of vibration. Some properties playing a key role for this dihydrogen bonded system, such as the bond length, bond angle, and distribution of atomic charges, are investigated in detail by comparing the results of path integral and classical molecular dynamics with those of the equilibrium structure. It was found that the atomic charges of Hdelta+ and Hdelta- are decreased and the dihydrogen Wdelta+...Hdelta- bond length is expanded as the thermal and zero-point quantum effects. (c) 2005 Elsevier B.V. All rights reserved.