Polarized-orbital valence-bond calculations on the ground state properties of H5+

Ab initio calculations were made on ${\mathrm{H}}_5^{+}$ in order to predict its equilibrium geometry, binding energy, vibrational frequencies, and enthalpy of formation. The method was based on a valence‐bond configuration interaction wavefunction built from linear combinations of Gaussian basis orbitals and polarized by Sambe's method. Optimization of nonlinear parameters was handled by a conjugate directions procedure derived from Powell's and Zangwill's work. ${\mathrm{H}}_5^{+}$ was found to have a C_2ν geometry in agreement with Esterfield and Linnett and with Huang, Schwartz, and Pfeiffer. The binding energy was estimated to be approximately D_e = 0.1 eV and the enthalpy of formation from ${\mathrm{H}}_3^{+}$ and H₂ to be about ΔH = − 3.3 kcal/mole, which is the same order of magnitude as experimental measurements. The importance of doubly occupied structures and of polarization was assessed from our wavefunctions.