This variable selects which Poisson solver
is used to compute the Coulomb integrals over a submesh.
These are non-periodic Poisson solvers.
If the domain parallelization is activated, the default is the direct sum.
Otherwise, the FFT Poisson solver is used by default.
Options:
dft_u_poisson_direct:
(Default) Direct Poisson solver. Slow.
dft_u_poisson_isf:
(Experimental) ISF Poisson solver on a submesh.
This does not work for non-orthogonal cells nor domain parallelization.
dft_u_poisson_psolver:
(Experimental) PSolver Poisson solver on a submesh.
This does not work for non-orthogonal cells nor domain parallelization.
Requires the PSolver external library.
dft_u_poisson_fft:
FFT Poisson solver on a submesh.
This uses the 0D periodic version of the FFT kernels.
This is not implemented for domain parallelization.