Do numerical MHD codes contain particles? Does an MHD code contain charged particles, or does one just numerically solve the MHD equations? Also, can you suggest a good introductory book/paper to learn the basics of MHD codes?
 A: 
Do numerical MHD codes contain particles?

No, magnetohydrodynamic (MHD) codes do not contain particles.  They are fluid simulations and so only track velocity moments.  In an ideal MHD simulation, the plasma is treated as a single fluid.  In Hall MHD, the simulation can be written with two fluids, ions and electrons.

Does an MHD code contain charged particles, or does one just numerically solve the MHD equations?

Well yes, an MHD code does solve the MHD equations but it's not a trivial thing to do.  The code is resolved on a user-defined mesh/grid.  The programmers need to do a lot of things to avoid flux pile-up, energy conservation, etc.

Also, can you suggest a good introductory book/paper to learn the basics of MHD codes?

A good reference on the merger between particle-in-cell (PIC) and fluid MHD codes can bef ound at:
https://ui.adsabs.harvard.edu/abs/2014JCoPh.276..468W/abstract
Here are some decent starting points (and references therein) on MHD codes:

*

*https://ui.adsabs.harvard.edu/abs/2005JCoPh.208..315M/abstract

*https://ui.adsabs.harvard.edu/abs/1981JCoPh..43..357R/abstract

*https://ui.adsabs.harvard.edu/abs/1995ApJ...442..228R/abstract
