= Neutral Coulomb pair in a uniform magnetic field
{title2=$\mathbf K=M\dot{\mathbf R}-e\mathbf B\times\mathbf r$}
For charges $+e,-e$ and relative coordinate $\mathbf r=\mathbf r_2-\mathbf r_1$, total magnetic pseudomomentum is conserved in a uniform <magnetic field>. In an appropriate symmetric-gauge <Lagrangian> differing by a <total derivative>, it is the canonical centre-of-mass <momentum>. Mechanical centre-of-mass <momentum> instead equals $\mathbf K+e\mathbf B\times\mathbf r$ and need not be constant. The sector $\mathbf K=0$ is not generally a stationary-centre-of-mass frame.
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