$v$). This statement becomes evident if we apply the scaling rules of NRQCD [@4]: $$\Psi_Q\sim (m_Qv_Q)^{\frac{3}{2}},\quad|\boldsymbol{ D}|\sim
m_Qv_Q,\quad gE\sim m_Q^2v_Q^3,\quad gB\sim m_Q^2v_Q^4,\quad g\sim
v_Q^{\frac{1}{2}}.$$ For the interaction of heavy quark with the light one, there is no such small velocity parameter, so that the Darwin term is suppressed by the additional factor of $k/m_Q\sim
\Lambda_{QCD}/m_Q$.

Further, the phenomenological experience with the potential quark models shows, that the kinetic energy of quarks practically does not depend on the quark contents of system, and it is determined by the color structure of state. So, we suppose that the kinetic energy is equal to $T = m_dv_d^2/2 + m_lv_l^2/2$ for the quark-diquark system, and it is $T/2 = m_{b}v_{b}^2/2 +
m_{c}v_{c}^2/2$ in the diquark (the color factor of 1/2). Then $$\frac{\langle
\Xi_{QQ'}^{\diamond}|\Psi_Q^{\dagger}(i\boldsymbol{D})^2\Psi_Q|\Xi_{QQ'}^{
\diamond} \rangle }{2M_{\Xi_{QQ'}^{\diamond}}m_Q^2}\simeq
v_Q^2\simeq
\frac{2m_qT}{(m_q+m_{Q'}+m_{Q})(m_{Q'}+m_{Q})}+\frac{m_{Q'}T}{m_Q(m_Q+m_{Q'
})},$$ $$\frac{\langle
\Xi_{QQ'}^{\diamond}|\Psi_{Q'}^{\dagger}(i\boldsymbol{D})^2\Psi_{Q'}|\Xi_{QQ'}^
{
\diamond} \rangle }{2M_{\Xi_{QQ'}^{\diamond}}m_{Q'}^2}\simeq
v_{Q'}^2\simeq
\frac{2m_qT}{(m_q+m_{Q'}+m_{Q})(m_{Q'}+m_{Q})}+\frac{m_QT}{m_{Q'}(m_Q+m_{Q'})},$$ where the diquark terms dominate certainly. Applying the quark-diquark approximation and relating the matrix element of chromomagnetic interaction of diquark with the light quark to the mass difference between the exited and ground states $M_{\Xi_{QQ'}^{\diamond *}} - M_{\Xi_{QQ'}^{\diamond}}$, we have $$\begin{aligned}
\frac{\langle \Xi_{cc}^{\diamond}|\bar cc|\Xi_{cc}^{\diamond}\rangle
}{2M_{\Xi_{cc}^{\diamond}}} &=& 1 -
\frac{1}{2}v_c^2 -
\frac{1}{3}\frac{M_{\Xi_{cc}^{\diamond *}}-M_{\Xi_{cc}^{\diamond}}}{m_c}
- \frac{5 g^2}{18 m_c^3}|\Psi (0)|^2 