Selects the method to perform subspace diagonalization. The
default is standard, unless states parallelization is used,
when the default is scalapack.
Note that this variable is not parsed in the case of the evolution eigensolver.
No subspace diagonalization. WARNING: this will generally give incorrect results.
The standard routine. Can be used with domain parallelization but not
State-parallelized version using ScaLAPACK. (Requires that
Octopus was compiled with ScaLAPACK support.)