Obtaining the right orbitals is the first step to calculating accurate binding energies for Cu⁺ ion

We investigate the previously reported poor performance of the G2 method for simple copper systems. We optimized CuH⁺, CuO⁺, and CuSi⁺ using the HF, MP2, B3LYP, mPW1PW91, and MPW1K methods with three basis sets. We found multiple solutions to the Hartree-Fock equations, which are the cause for previously reported poor behavior of the G2 method. The orbitals of the lowest-energy unrestricted Hartree-Fock solution do not always generate the lowest-energy correlated wavefunction. Hybrid density functional theory methods are shown to be quite successful for obtaining orbitals and for predicting geometries and atomization energies.