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Conjugate gradient optimization

Instead of the previous iteration scheme, which is just
some kind of Quasi-Newton scheme, it also possible to optimize the
expectation value of the Hamiltonian using a successive number of
conjugate gradient steps.
The first step is equal to the steepest descent step in section 7.1.3.
In all following steps the preconditioned gradient
is conjugated to the previous search direction.
The resulting conjugate gradient algorithm is almost as efficient as the algorithm
given in section 7.1.4.
For further reading see [20,21,28].

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