work and those of Ref. [@Li:2015zda] is collected in Table \[tab:improvements\].

[l|ccccc]{} & $\overline{\delta_j M}_{c\bar c}$ & $\delta M_{c\bar c}$ (rms) & $\overline{\delta_j M}_{b\bar b}$ & $\delta M_{b\bar b}$ (rms) & $N_{\max} = L_{\max}$\
fix-$\alpha_s$ [@Li:2015zda] & 49 MeV & 52 MeV & 17 MeV & \*58 MeV & 24\
fix-$\alpha_s$ (refitted) & — & — & 15 MeV & 48 MeV & 24\
running-$\alpha_s$ & 17 MeV & 31 MeV & 7 MeV & 39 MeV & 24\
running-$\alpha_s$ & 17 MeV & 31 MeV & 8 MeV & 38 MeV & 32\

\
[\* In Ref. [@Li:2015zda], this is misquoted as 50 MeV.]{}

Our light-front Hamiltonian approach yields states with high angular and radial excitations, which are not easily accessible in some other methods. No exotic quantum numbers emerge from our calculation, as is expected from the two-body truncation. In bottomonium, predictions are made for various states below the $B\overline B$ threshold, as also predicted in other approaches (e.g., [@Crater:2010fc; @McNeile:2012qf; @Hilger:2014nma]). The quality of the spectra can be measured by the root mean squared (r.m.s.) deviation from the experimentally measured values. For charmonium (bottomonium), the r.m.s. mass deviation is 31 MeV (38 MeV), improving the fixed coupling 