A full-dimensional, ab initio based potential energy surface (PES) for CH5+, which can describe dissociation is reported. The PES is a precise fit to 36173 coupled-cluster [CCSD(T)] calculations of electronic energies done using an aug-cc-pVTZ basis. The fit uses a polynomial basis that is invariant with respect to permutation of the five H atoms, and thus describes all 120 equivalent minima. The rms fitting error is 78.1 cm(-1) for the entire data set of energies tip to 30000 cm(-1) and a normal-mode analysis of CH5+ also verifies the accuracy of the fit. Two saddle points have been located on the surface as well and compared with previous theoretical work. The PES dissociates correctly to the fragments CH3+ + H-2 and the equilibrium geometry and normal-mode analyses of these fragments are also presented. Diffusion Monte Carlo calculations are done for the zero-point energies of CH5+ (and some isotopologs) as well as for the separated fragments of CH5+, CH3+ + H-2 and those of CH4D+, CH3+ + HD and CH2D+ + H-2. Values of D-0 are reported for these dissociations. A molecular dynamics calculation of CH4D+ dissociation at one total energy is also performed to both validate the applicability of the PES for dynamics studies as well as to test a simple classical statistical prediction of the branching ratio of the dissociation products.