Ab initio, calculations have been carried out at MP2 with the 6-31+G(d,p), 6-311+G(d,p), aug'-cc-pVDZ, aug'-cc-pVTZ, aug-ccpVTZ, and aug'-cc-pVQZ basis sets, and at MP4, CCSD, and CCSD(T) with the aug'-cc-pVTZ basis, to probe the potential surfaces for equilibrium structures of HCPH+. All levels of theory indicate that protonation occurs at C, with the formation of pi-protonated C-s(pi) Or C-2v structures. The non-classical C-s(pi) structure is usually preferred, although the potential surfaces are relatively flat in this region. (c) 2006 Elsevier B.V. All rights reserved.