Fix pressure tensor sign and ordering conventions#5398
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Co-authored-by: Jean-Noël Grad <jgrad@icp.uni-stuttgart.de>
test_dpd_stress originally looped over kT=[0, 2], comparing the thermalized stress against a dissipative-only reference. That reference omits the random force, so it is only valid before the noise term was added to the DPD stress calculation. Restrict test_dpd_stress to the unthermalized case (kT=0) and cover the noise term separately. Add test_dpd_stress_noise_statistics: a single fixed pair with zero relative velocity produces a pure-noise stress whose mean is zero and whose per-component variance follows the fluctuation-dissipation result Var(sigma_ij) = (d_i^2/V^2) * (1/12) * ((A^2-B^2)*dhat_j^2 + B^2). A generic off-axis separation makes all nine components nonzero and distinct, and sampling over many RNG counters checks them against the analytic values. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
The DPD virial was exposed under two names with opposite signs: dpd_stress()/DPDStress returned -sum(r x F)/V while the pressure tensor's dpd contribution returned +sum(r x F)/V. Retire the stress naming: system.analysis.dpd_pressure() and observables.DPDPressure now return the pressure convention (+, including the noise term), identical to pressure_tensor()["dpd"]. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Replace all instances of F.d^T with d.F^T
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Stacked on #5332 — the first 6 commits belong to that PR; only the last
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f7bb15b,cbb8799,4f98fdb,c0851d2) are new here.Description of changes: