CCP4 web logo Basic Maths for Protein Crystallographers
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The length of the reciprocal lattice vector ha*+kb*+lc* is called d* and equals 2sinthetaoverlambda.
Crystallographers often define |S|² as (d*)² or 4sinsqthetaoverlambdasq. S, a vector, is defined as s-s0, where s is in the direction of the scattered wave, and s0 is in the direction of the incident wave.
Protein crystallographers talk about resolution in terms of 1/d* (or d), hence the 'high resolution' limit is actually a small number.