Add noise term in dpd pressure calculation#5332
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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>
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Co-authored-by: Jean-Noël Grad jgrad@icp.uni-stuttgart.de
Fixes #
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