The Davidson iteration scheme optimizes all bands simultaneously. Optimizing a single
band at a time would save the storage necessary for the NBANDS gradients.
In a simple steepest descent scheme the preconditioned residual vector 
is orthonormalized
to the current set of wavefunctions i.e.
Then the linear combination of this 'search direction' 
and the current wavefunction 
is calculated which minimizes the expectation value of the Hamiltonian.
This requires to solve the 
eigenvalue problem
with the basis set
