My memory is that the idea is related to the smoothness of the metric/gauge terms: these should be C^2, so are sufficiently differentiable to be "best" approximated (at the second order that Whisky/GRHydro worked to) by the midpoint rule. However, if included within the fluxes then they would implicitly go through the reconstruction/Riemann solve stage, so would - in places - be limited to lower accuracy.

I would not do this now - keeping the fully densitized terms together is cleaner and a better way to extend to high order.

Ian

From: Roland Haas <rhaas@illinois.edu>
Sent: 31 October 2021 20:28
To: Ian Hawke <I.Hawke@soton.ac.uk>
Cc: carpetx-developers@einsteintoolkit.org <carpetx-developers@einsteintoolkit.org>
Subject: Re: lapse terms in Whisky's fluxes