MODFLOW 6 is parallelized using the Message Passing Interface (MPI) and the Portable, Extensible Toolkit for Scientific Computation (PETSc). This HOWTO explains how to compile these dependencies and MODFLOW 6 on Slackware.

# switch to root
su -
 
# install openmpi and python3-numpy; petsc requires python3-numpy;
# build queuefiles, not packages, to automatically build any other dependencies
sbopkg -k -i "openmpi python3-numpy" # hit Q when prompted and Proceed

Download petsc.zip.