From 738d7151f127945d20d49a72944336a790c54ee6 Mon Sep 17 00:00:00 2001
From: Chad Mitchell <46825199+cemitch99@users.noreply.github.com>
Date: Thu, 19 Sep 2024 17:02:59 -0700
Subject: [PATCH] Fix special case of pt_ms=0 in dispersive diagnostics. (#707)

---
 src/particles/diagnostics/ReducedBeamCharacteristics.cpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/particles/diagnostics/ReducedBeamCharacteristics.cpp b/src/particles/diagnostics/ReducedBeamCharacteristics.cpp
index 0b6ed024d..3b4d19ffd 100644
--- a/src/particles/diagnostics/ReducedBeamCharacteristics.cpp
+++ b/src/particles/diagnostics/ReducedBeamCharacteristics.cpp
@@ -262,10 +262,10 @@ namespace impactx::diagnostics
         amrex::ParticleReal const emittance_y = std::sqrt(y_ms*py_ms-ypy*ypy);
         amrex::ParticleReal const emittance_t = std::sqrt(t_ms*pt_ms-tpt*tpt);
         // Dispersion and dispersive beam moments
-        amrex::ParticleReal const dispersion_x = - xpt / pt_ms;
-        amrex::ParticleReal const dispersion_px = - pxpt / pt_ms;
-        amrex::ParticleReal const dispersion_y = - ypt / pt_ms;
-        amrex::ParticleReal const dispersion_py = - pypt / pt_ms;
+        amrex::ParticleReal const dispersion_x = ((pt_ms > 0.0) ? (- xpt / pt_ms) : 0.0);
+        amrex::ParticleReal const dispersion_px = ((pt_ms > 0.0) ? (- pxpt / pt_ms) : 0.0);
+        amrex::ParticleReal const dispersion_y = ((pt_ms > 0.0) ? (- ypt / pt_ms) : 0.0);
+        amrex::ParticleReal const dispersion_py = ((pt_ms > 0.0) ? (- pypt / pt_ms) : 0.0);
         amrex::ParticleReal const x_msd = x_ms - pt_ms*dispersion_x*dispersion_x;
         amrex::ParticleReal const px_msd = px_ms - pt_ms*dispersion_px*dispersion_px;
         amrex::ParticleReal const xpx_d = xpx - pt_ms*dispersion_x*dispersion_px;