heavy baryons [@DHD1; @DHD2], so we omit it here.

Further, employing the NRQCD expansion of operators $\bar Q Q$ and $\bar Qg\sigma_{\mu\nu}G^{\mu\nu}Q$, we have $$\begin{aligned}
\bar QQ &=& \Psi_Q^{\dagger}\Psi_Q -
\frac{1}{2m_Q^2}\Psi_Q^{\dagger}(i\boldsymbol{ D})^2\Psi_Q +
\frac{3}{8m_Q^4}\Psi_Q^{\dagger}(i\boldsymbol{ D})^4\Psi_Q
-\nonumber\\ &&
\frac{1}{2m_Q^2}\Psi_Q^{\dagger}g\boldsymbol{\sigma}\boldsymbol{
B}\Psi_Q - \frac{1}{4m_Q^3}\Psi_Q^{\dagger}(\boldsymbol{
D}g\boldsymbol{ E})\Psi_Q + ... \label{32}\\ \bar
Qg\sigma_{\mu\nu}G^{\mu\nu}Q &=&
-2\Psi_Q^{\dagger}g\boldsymbol{\sigma}\boldsymbol{ B}\Psi_Q -
\frac{1}{m_Q}\Psi_Q^{\dagger}(\boldsymbol{ D}g\boldsymbol{
E})\Psi_Q + ... \label{33}\end{aligned}$$ Here the factorization at scale $\mu$ ($m_{Q} >  \mu > m_{Q}v_{Q}$) is supposed. We have omitted the term of $\Psi_Q^{\dagger}\boldsymbol{\sigma}
(g\boldsymbol{E} \times \boldsymbol{ D})\Psi_Q$, corresponding to the spin-orbital interactions, which are not essential for the basic state of baryons under consideration. The field $\Psi_Q$ has the standard non-relativistic normalization.

Now we would like to make some comments on the difference between the descriptions of interactions of the heavy quark with the light and heavy heavy ones. As well known, in the doubly heavy subsystem there is an additional parameter which is the relative velocity of quarks. It introduces the energy scale equal to $m_Q v$. Therefore, the Darwin term ($\boldsymbol{ D}\boldsymbol{ E}$) in the heavy subsystem stands in the same order of inverse heavy quark mass in comparison with the chromomagnetic term ($\boldsymbol{\sigma}\boldsymbol{ B}$) (they have the same power in the velocity 