@wds15 Thank you for sharing the article, I’ll dive into it.
I’m linking the article which compares various autodiff methods for differential equations in Julia, with some examples from the PKPD litterature: https://arxiv.org/pdf/1812.01892.pdf.
Ideally, Stan would let the user choose which differentiation method to use (forward, adjoint, and maybe more). @bbbales2, I remember you were working on implementing the adjoint method in Stan and per this last thread, things seemed to be moving forward pretty smoothly: Adjoint task force for ODEs Are there any updates on this?