International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2010). Vol. B, ch. 3.4, p. 454   | 1 | 2 |

## Section 3.4.7. The cases of n = 2 and n = 3

D. E. Williamsa

aDepartment of Chemistry, University of Louisville, Louisville, Kentucky 40292, USA

### 3.4.7. The cases of n = 2 and n = 3

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If the denominator considered for the limit in the preceding section is linear in |H| so that only one differentiation is needed to obtain the limit by L'Hospital's method. Since a term of the type is always a factor, the requirement that the unit cell have no dipole moment can be relaxed. For the zero-charge condition is still required: . When the expression becomes determinate and no differentiation is required to obtain a limit. In addition, factoring the sums into sums is not necessary so that the only remaining requirement for this term to be zero is , which is a further relaxation beyond the requirement of cell neutrality.