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**Up:** An Introduction to Direct Methods. The
** Previous:** The Positive Quartet Relation

In the event that the sum of the four phases is equal to :

(34) |

The planes of maximum for the 4 reflection involved in relation (34) are
indicated in Fig. 16. It can be seen that for all indicated positions three out
of four planes intersect. If atoms are located at these points the resulting
unitary structure factors of *H*, *K*, *L* and -*H*-*K*-*L* will be 0.5, because three
atoms lie in the planes and one lies halfway between. Thus for a negative
quartet relation the reflections *H*, *K*, *L* and -*H*-*K*-*L* will in general not be
found amongst the very strongest. The next question to be answered is: what is
the intensity of reflection *H* + *K*, if the electron density is located near the
marked points of Fig. 16? From Fig. 17 it can be easily seen that *H* + *K* will
have a small |*E*|-magnitude: equal numbers of points of electron density
concentration lie on the *H* + *K* planes and halfway in between. As the same
holds for the other cross terms it can be stated that the negative quartet
relation (34) is likely to be true for reasonably large values of *E _{4}* and
small |

**Copyright © 1984, 1998 International Union of
Crystallography**