Convergence problems in lattice sums

The problem of convergence of coulombic sums can be acute in crystals with polar space groups, when molecular dipoles are parallel or nearly parallel to the polar axis. Indeed, in such cases even the physical meaning of a coulombic energy is questionable. The problem can be approximately solved by the estimation of an additional term for the coulombic energy, derived by an integration over a uniform distribution of dipolar unit cells [22]  

Ucom — I2sums 1/(4JtS )2k/7 /(3Vcell) = t/sums - 2909.9pV(ZVcell) 

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