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Carry the Jacobian sign in the Mercier volume derivative #795

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Carry the Jacobian sign in the Mercier volume derivative #795

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9c90918
Carry the Jacobian sign in the Mercier volume derivative
CharlesCNorton Sep 4, 2026
4a569d6
Merge branch 'main' into mercier-jacobian-sign
CharlesCNorton Sep 4, 2026
369afe4
Rerun CI
CharlesCNorton Sep 4, 2026
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5 changes: 4 additions & 1 deletion src/vmecpp/cpp/vmecpp/vmec/output_quantities/output_quantities.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -3187,9 +3187,12 @@ vmecpp::MercierFileContents vmecpp::ComputeMercierStability(
// S
mercier.s[jF] = mercier_intermediate.s[jF];

// mercier.f90 forms this as sqs = 0.5*(vp_real(i) + vp_real(i+1))*sign_jac
// and divides SHEAR, ITOR' and PRES' by it, so the Jacobian sign belongs
// here rather than only on WELL.
const double vp_full = (mercier_intermediate.vp_real[jHo] +
mercier_intermediate.vp_real[jHi]) /
2.0;
2.0 * vmec_internal_results.sign_of_jacobian;
if (vp_full == 0.0) {
// skip this surface
continue;
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2 changes: 1 addition & 1 deletion src/vmecpp/cpp/vmecpp_large_cpp_tests/vmec/output_quantities/output_quantities_test.cc
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Expand Up @@ -671,7 +671,7 @@ TEST_P(MercierStabilityTest, CheckMercierStability) {

const double vp_full = (static_cast<double>(mercier["vp_real"][jHo]) +
static_cast<double>(mercier["vp_real"][jHi])) /
2.0;
2.0 * sign_of_jacobian;

// The running sum advances even on surfaces the assembly skips, and is
// scaled by deltaS only at the end, as the assembly does.
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