The geometry and relative energies of torsional conformers of centrally protonated C4H11+ were studied with ab initio methods, to (a) obtain the most accurate geometry of the three-center-two-electron CHC bond to date, (b) evaluate the performance of lower levels of approximation upon this challenging structure, and (c) gain an understanding of the torsional dynamics of C4H11+. Twenty-nine combined levels of theory were used to optimize the geometry of the C-2-symmetry minimum for trans-C4H11+, and the most accurate one [CCSD(T)/cc-pVTZ] gave the following CHC bond geometry: theta(CHC)=122.4degrees, R-CC=2.177 Angstrom, R-CH=1.2424 Angstrom. Molecular-orbital-based methods generally perform better than density functional methods for describing the three-center-two-electron bond. A smaller subset of levels of theory was used to optimize other torsional conformers of centrally protonated C4H11+, varying the CCCC dihedral (trans, gauche, cis) and the dihedral for the bridging proton (various eclipsed and staggered positions). The results show that all conformers lie within a 4 kJ mol(-1) range, with the lowest-energy conformer being either trans or gauche with a staggered dihedral for the bridging proton. The effect of core-valence correlation was also investigated. Finally, the potential energy surface as a function of the CCCC and bridging-proton dihedral angles was qualitatively estimated and drawn, based on our computed data, to aid in understanding the fluxional character of C4H11+. (C) 2003 American Institute of Physics.