12. Search engines¶
Indexing establishes what a peak list can say about a lattice. The search is how a cell is found in the first place.
Two engines are implemented. They share the Q form, the tolerance model and the scoring panel, and nothing else: one exhausts a metric domain, and the other guesses indices and solves. Both source papers conclude that no single indexing program wins, and that running several raises the success rate [Bergmann et al., 2004]. That is the device this package uses for confidence elsewhere. Agreement between independent methods is the evidence, and disagreement is reported rather than averaged.
12.1. Searching in the metric components¶
Both published methods this chapter follows work in direct cell parameters. Here the search coordinates are the metric components of (11.1) instead, because \(Q\) is linear in them. Over an axis-aligned box \([\theta^-, \theta^+]\) the extremes of \(Q(hkl)\) are therefore attained exactly at corners and are read off componentwise,
Source: rietx.indexing.dichotomy._q_bounds
with \(B\) the metric basis of (11.4) and \(\mathbf{d}(hkl) = (h^2, k^2, l^2, kl, hl, hk)\) the design row of (11.2). No corner is enumerated, and there are \(2^n\) of them. The bound is tight rather than merely valid, which matters because a loose bound prunes less while still returning correct cells, a regression no recovery test would notice.
The direct-space formulation has no such property. Its \(Q\) is a ratio of trigonometric expressions in the parameters, so the 1991 method needs an eight-case monotonicity analysis for the cross terms when \(hl < 0\), and its authors attribute their own pre-1991 monoclinic cost to having got those bounds loose [Boultif and Louër, 1991]. In \((A \ldots F)\) that case split does not exist in any system, triclinic included.
The box lives in the symmetry subspace, so its dimension is the metric degrees of freedom, 1 for cubic and 6 for triclinic. The coordinates \(\theta\) are read off the basis’s pivot structure.
12.2. Successive dichotomy¶
The method divides a parameter domain into subdomains, discards any that
provably cannot contain a solution, and recurses
[Boultif and Louër, 1991; Boultif and Louër, 2004; Louër and Louër, 1972]. Its contrapositive is what earns its
cost next to a cheaper engine. When it exhausts a domain and finds nothing, no
cell of that symmetry within those bounds fits the peak list. No other method
here can make that statement, so search_complete is reported per system rather
than assumed. An unfinished search has said nothing at all, and the two cases
must not look alike downstream.
The domain is set by the principal \(d\)-spacings on the diagonal components and by an obliquity bound on the rest,
Source: rietx.indexing.dichotomy._initial_box
the second being Cauchy–Schwarz with the reciprocal angles restricted to \([\vartheta_{\max}, 180° - \vartheta_{\max}]\). It is reported as a bound of the search, never as a claim about lattices. A cell more oblique than this is outside the domain, and its absence from the results means nothing.
Three prunes run per box, cheapest first. Each is monotone under bisection: a child’s intervals are subsets of its parent’s. That monotonicity makes the pruning sound.
12.2.1. The metric cone and the volume window¶
A box whose diagonal interval is non-positive is not a lattice, and \(\det G^*\) over the box must intersect the band \([V_{\max}^{-2}, V_{\min}^{-2}]\) implied by the declared volume range. Bounding that determinant by interval arithmetic on its expanded form is correct and nearly useless: it evaluates \(-G^*_{22}(E/2)^2\) with \(E\) at the domain’s maximum while the diagonals are at this box’s, and the term then swamps \(G^*_{11}G^*_{22}G^*_{33}\). Factoring the correlation matrix out instead keeps the coupling (12.2) already declares,
Source: rietx.indexing.dichotomy._det_interval
so that for a monoclinic box \(\det R \in [1 - \cos^2\vartheta_{\max}, 1]\) and the “this cell is too small” half of the test can fire at all.
12.2.2. Line matching and Hall’s condition¶
For each observed line, some trial \(hkl\)’s \([Q^-, Q^+]\) must reach it within \(k\sigma_{\text{eff}}\), and past the tolerated number of misses the box is impossible. That test alone is the weakest necessary condition there is, because it lets several lines point at the same reflection. An indexing is an injective map from lines to reflections, since one \(hkl\) has one \(Q\) and two resolved lines cannot both be it. The box must therefore admit a matching, and Hall’s theorem requires \(\lvert N(S)\rvert \ge \lvert S \rvert\) for every set \(S\) of lines. Two instances of that are free to check: \(S\) = every line, whose neighbourhood is the count of reflections still reaching anything, and \(S\) = the lines with a single candidate, where two lines forced onto one reflection is a violation at \(\lvert S \rvert = 2\).
12.2.3. One grid pass per system¶
A centred trial set is a strict subset of the primitive one, so a centred box has fewer reflections with which to reach the same lines. Its matching test is strictly harder, and every box surviving it survives the primitive test. The centred pass can therefore find no metric the primitive pass misses. All it ever contributed was the scoring, recovered by re-running the assignment under each admissible centring at the leaves.
The search runs in two phases, and merging them into one loop breaks it. A merged depth-first stack dives to a leaf through the first grid cell it likes and explores that cell’s entire bisection subtree, which in four dimensions is effectively unbounded, before it looks at the second cell. So the grid is completed breadth-first at the literature’s steps (0.4 Å in a principal \(d\)-spacing and 5.0° in a reciprocal angle [Altomare et al., 2019]), with the prunes applied between dimensions, and only then are the survivors bisected, best first.
Where to stop bisecting is a separate question from the measurement tolerance, and conflating the two breaks the search outright. Exhaustiveness comes from the pruning, which is exact however coarse the leaves are. Acceptance only has to leave a box whose centre the assign-and-refine loop can polish onto the true cell. The box is small enough when the indexing inside it is unique, with no observed line carrying two candidate reflections. A width crossing a threshold is the wrong test, because a high-index reflection has a large \(\lVert m \rVert\) and its interval stays wide long after the assignment has stopped being ambiguous.
Tolerated unindexed lines are the most valuable option here, and the gain the 2004 paper reports for itself [Boultif and Louër, 2004]. Without them one impurity line prunes the box containing the truth and the engine returns nothing, confidently, which is the failure this whole subsystem exists to prevent. Raising the count past a small default manufactures cells instead: each extra tolerated line is one more coincidence a wrong metric is allowed to have.
12.3. Index-heuristic trial and error¶
The second engine assumes the indices of a few base lines and solves the metric exactly [Werner, 1964; Werner et al., 1985]. With \(n\) metric parameters, \(n\) assigned lines determine them by an exact \(n \times n\) solve, so the search runs over index assignments rather than over a domain. That is a combinatorial space no volume window prunes, and one where a bad base line poisons the answer exactly where a wide domain poisons the dichotomy. It is the independence worth having: the two engines fail on different inputs.
Its base-line index limit is small for an arithmetic reason. A base line of low \(Q\) cannot carry a large index without implying an axis longer than the domain allows.
Source: rietx.indexing.trial_error.search_trial_error
The engine carries no exhaustiveness claim. It can contribute a found_by vote
and a Borda rank, and it can never contribute to search_complete. A restricted
search is never a verdict about the sample.
12.4. Zone indexing, assessed and declined¶
A third engine in the ITO lineage [Visser, 1969] was assessed and left unimplemented. The attractive half of the argument is real. ITO searches zones: the continuously searched dimension is never more than one at any stage, in any system including triclinic, so a monoclinic problem becomes three sequential one-dimensional coincidence searches rather than a four-dimensional box. It assumes no indices, only that two of the first few lines are real reflections whose common zone is populated. That is a genuinely different failure mode from both landed engines, and it is what would make an agreement vote worth something.
What refutes the hopeful version is that it buys no immunity to a zero-point error. With
a constant offset \(Q \to Q + \delta\) shifts the four \((m, n)\) branches by different multiples of \(\delta\), so it splits the coincidence peak instead of translating it. The acceptance test is a count, so splitting is worse than shifting, and per-line errors amplify. An engine with that property fails on exactly the uncalibrated laboratory data the systematic-error model of Indexing exists for. It also carries no exhaustiveness claim, and its reduction is weak above twofold symmetry, so it is an orthorhombic-and-below specialist.
12.5. The obliquity bound¶
\(\vartheta_{\max}\) is the one domain parameter with no physical answer. Every monoclinic lattice has a \(b\)-unique description whose \(\beta\) lies in \([60°, 120°]\), because the \(a\)–\(c\) plane is a two-dimensional lattice and Gauss reduction gives \(\lvert\cos\beta\rvert \le a/2c \le 1/2\). A narrow bound therefore loses some descriptions of a lattice and no lattice. Against that, the bound enters the cost twice: directly, through the number of angular slabs the grid cuts, and again through (12.3), where \(1 - \cos^2\vartheta_{\max}\) is the whole strength of the small-cell prune.
The bound is nevertheless kept wide, because the search’s setting conventions are prunes rather than projections, and a box straddling the axis-ordering constraint is kept. “The reduced description is somewhere in the domain” is then an argument about lattices that the implementation does not structurally enforce for every system. A search bound may be loose. The one thing it may not do is exclude the true cell.
12.6. The reflection ceiling¶
Every leaf is checked before any enumeration. The count of reflections a cell would generate to a given \(2\theta\) is arithmetic on the cell, so the guard costs nothing and never touches memory. That is the point: the cells a search proposes include ones no physical specimen has, and a refinement free to move produced a 49 Å axis at \(\beta = 174°\) in testing.