A formula for the invariant cos(ϕ_h + ϕ_k + ϕ_l – ϕ_h+k+l) in the procedures for phase solution

Several random structures are investigated to test formulae given in the preceding paper [Giacovazzo (1976). Acta Cryst. A32, 91-99]. As the values of the cosine invariant cos (φ_h + φ_k + φ_l - φ_{h + k + l}) are generally overestimated, an empirical scaling factor is introduced. The reliability of the phase indications in quartets is then similar to that in triplets, justifying the simultaneous use of triplets and quartets in tangent procedures for phase determination.