Performance of DFT functionals for properties of small molecules containing beryllium, tungsten and hydrogen

A comparative study of bond lengths, atomization energies and vibrational frequencies for a set of neutral molecules Be-n, BenHm, W-n,WnBem, and WnHm with m + n <= 4 in their ground state is presented. We compare 16 density functionals chosen from rungs 1 to 4 of Jacob's ladder and Hartree-Fock results to Coupled Cluster calculations. For atomization energies, omega B97XD, closely followed by B97D, M06, B3LYP and M11, shows the best performance while M11, omega B97XD and HSEH1PBE are most faithful in reproducing bond lengths. The ranking sequence for predicting vibrational frequencies is HSEH1PBE, B3LYP and M11. The range-separated hybrid mega-GGA functional M11 stands out to be accurate for all three properties for the studied Be/H/W molecules. The basis set and size dependence of the CCSD(T) energies and the influence of inner core electrons on the atomization energies of several tungsten-containing molecules are also discussed. CCSD(T)/cc-pVQZ energies are already close to the complete basis set limit from a cc-pVQZ/cc-pV5Z extrapolation level. Core correlation contributes with 3-5% for W-2 and W hydrides in energies. The comparison of DFT functionals in this particular setting and the set of molecules allows to evaluate the accuracy of atomistic plasma-wall interaction studies for the ITER environment, where Be, W, and H/D/T interact with each other.