The work carried out in this study involves a considerable amount of high-performance computing (HPC) resources, close to two million computational hours. Most of the computational effort is due to the large number of steps required to avoid imaginary frequencies in the optimization process for the high theory level ωB97X/6-311++G(d,p). Previous work showed that zeolitic structures having imaginary modes not only exhibit higher energies than those with all real modes, but also do not compare well with the results from high resolution experimental techniques. We have observed that clusters with Im -have a flatter energy surface than clusters with mIm -, resulting in slow convergence: in some cases the computational time for geometry relaxations and frequency calculations has been as long as months using 24 CPUs, -i.e. the Zn 4 Im 12 cluster or Zn 6 Im 18 , and more than 4 months with 40 CPUs were spent trying to optimize the Zn 8 Im 20 (D4R) cluster without success.