a natural dependence on the covariant 4-momentum transfer $Q^2$. The use of the running coupling also serves to improve the UV asymptotics of the one-gluon exchange kernel. In our previous work [@Li:2015zda], we used a fixed coupling. The effective one-gluon exchange kernel, as derived from the leading-order effective Hamiltonian approach, produces a divergent results, as is well known in the literature (e.g., Refs. [@Krautgartner:1991xz; @Gubankova:1997mq; @ManginBrinet:2003nm]). This divergence is the result of the high momentum contribution from the spin non-flip part of the Hamiltonian matrix elements. It can be easily seen from the power counting in transverse momenta. In Ref. [@Li:2015zda], we adopted a UV counterterm proposed by Krautgärtner, Pauli and Wölz (KPW) [@Krautgartner:1991xz] (cf. Refs. [@Glazek:1992bs; @Trittmann:1997xz; @Wiecki:2014ola; @Lamm:2016djr]). However, the KPW counterterm is non-covariant, and introduces a major source of violation of the rotational symmetry that is manifested in the spectrum. With asymptotic freedom, the UV divergence associated with the one-gluon exchange kernel is absent. Therefore, the non-covariant KPW counterterm is not needed and we omit it in the present work. As we will see below, the rotational symmetry is improved compared to the results of Ref. [@Li:2015zda].

Hamiltonian formalism {#sect:formalism}
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Eigenvalue Equation
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The mass spectrum 