About c++ : How-to-choose-MPI-vendordistribution-with-CMake

I have a program that I would like to compile with CMake + make but using two different MPI distributions, OpenMPI and MPICH.

In Ubuntu, I have both installed; these are all the compiler wrappers I have installed:

mpic++           mpicxx           mpif77.mpich     mpijavac       
mpicc            mpicxx.mpich     mpif77.openmpi   mpijavac.pl    
mpiCC            mpicxx.openmpi   mpif90           mpirun         
mpicc.mpich      mpiexec          mpif90.mpich     mpirun.mpich   
mpicc.openmpi    mpiexec.hydra    mpif90.openmpi   mpirun.openmpi 
mpiCC.openmpi    mpiexec.mpich    mpifort          mpivars        
mpichversion     mpiexec.openmpi  mpifort.mpich                   
mpic++.openmpi   mpif77           mpifort.openmpi   

(OpenMPI is the default, i.e. when no distribution extension is specified. I am not using modules.)

How can I force CMake to choose MPICH over OpenMPI?

I tried setting -DMPI_ROOT=/usr/lib/x86_64-linux-gnu/mpich but

I get this error:

-- Could NOT find MPI_CXX (missing: MPI_CXX_WORKS) 
CMake Error at /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find MPI (missing: MPI_CXX_FOUND)

      Reason given by package: MPI component 'C' was requested, but language C is not enabled.  MPI component 'Fortran' was requested, but language Fortran is not enabled.  

Call Stack (most recent call first):
  /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.18/Modules/FindMPI.cmake:1721 (find_package_handle_standard_args)
  CMakeLists.txt:11 (find_package)

it still uses OpenMPI.

This is how my CMakeLists.txt looks like:

cmake_minimum_required(VERSION 3.12.0)
project(myproject VERSION 0.1 LANGUAGES CXX)

#enable_language(C)  #uncommenting these doesn't help
#enable_language(Fortran)

enable_testing()
include(CTest)

find_package(MPI REQUIRED)

Setting -DMPI_ROOT= or -DMPI_HOME= didn’t work for me. It still uses the default in my system (OpenMPI).

What worked was to set -DMPI_EXECUTABLE_SUFFIX=.mpich, option which I found near the end of the documentation: https://cmake.org/cmake/help/latest/module/FindMPI.html.

Based on cmake documents, you should be able to select one by setting value of MPI_GUESS_LIBRARY_NAME.

Update: According to same document:

Try to find an MPI implementation that does not ship such a wrapper by guessing settings. Currently, only Microsoft MPI and MPICH2 on Windows are supported.