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Single band, steepest descent scheme
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.

(7.2) 
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
N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at