Abstract: In this paper, it is demonstrated that |E(k)E(h-k)| is a structure invari ant. On the basis of this knowledge, three models, which can be used to estimate the value of |E(k)E(h-k)| from normalized structure amplitudes |E|, are proposed. In particular, |E(k)E(h-k)|_(QE), which is deduced from the second neighborhood principle of quartet structure invariants, is an ideal estimation of |E(k)E(h-k)| with known phase lacks. It is expected that |E(k)E(h-k)|_(QE) will improve the function of super-solution phase equation in systematical trial and error method and increase the accuracy of the phase values to be determined.

Key words: Phase equation, Structure invariant, Direct method