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.