{"id":"t1-niggli-audit","title":"T1 Niggli Audit — Path-15 Cell-Representation Check (2026-07-24)","subtitle":"VALID: raw vs Niggli-reduced cells shift absolute energies ~39 eV but the barrier moves only 3.5 meV — the third recorded offset-cancellation instance; no anchor recomputation needed.","category":"validation","tags":["t1","niggli","audit","sparse-dft","offset-cancellation"],"source":"articles/docs/analysis/t1-niggli-audit.md","lang":"en","words":157,"readMinutes":1,"toc":[],"html":"<h1 id=\"niggli-audit-path-15-bi-li-s-mp-1222404-2026-07-24\">Niggli audit — path-15 (Bi-Li-S, mp-1222404), 2026-07-24</h1><p><strong>Question:</strong> are the anchors on the 34.4° rhombohedral cell grid-suspect? (GPAW fd warned &quot;results may be wrong&quot;.)</p>\n<p><strong>Method:</strong> recompute basin (img0) and saddle (img2) at adopted settings (h=0.20, Gamma) on the raw cell vs the Niggli-reduced cell (<code>ase.build.niggli_reduce</code>). Verdict threshold: barrier shift &gt;40 meV = grid-suspect.</p>\n<p><strong>Results:</strong></p>\n<ul>\n<li>raw img0: −430.7262 eV (matches campaign checkpoint exactly); raw img2: −429.6101 eV (matches checkpoint)</li>\n<li>Niggli img0: −469.7732 eV; Niggli img2: −468.6536 eV (cell representation shifts absolute energies ~39 eV)</li>\n<li><strong>Barrier raw: 1.1161 eV vs Niggli: 1.1196 eV → shift +3.5 meV</strong></li>\n</ul>\n<p><strong>Verdict: VALID.</strong> The skewed cell&#39;s barrier matches the Niggli-reduced barrier within 3.5 meV — the ~39 eV representation offset cancels in the energy difference. This is the third recorded instance of the offset-cancellation pattern (convention offsets in T1, settings offsets in amendment 02, cell-representation offsets here): barriers are robust to constant offsets; only wander matters. No anchor recomputation needed; path-15&#39;s data stands as-is.</p>\n"}