[Pw_forum] Access to 2nd order derivatives
marcel at physik.tu-berlin.de
Mon Mar 19 14:06:35 CET 2007
On Mon, 19 Mar 2007, Paolo Giannozzi wrote:
> On Mar 14, 2007, at 22:00 , Marcel Mohr wrote:
>> I wonder if it is possible to obtain the 2nd derivative of the total energy
>> for a user defined set of atomic coordinates (not a real but infinitely
>> small displacement from the ground state).
> in principle, it is, and once upon a time it was possible.
> Now there is the 'single mode calculation' (example 02),
> in which you can choose a specific displacement pattern,
> but only among those automatically computed by the code.
Thank you very much for your answer.
One can "move" one atom, and that in the directions x, y, and z.
The variable in pw.x modenum 3 is corresponds to a "displacement" of
the first atom in direction z.
ph.x computes then the dynamical matrix entries for this mode, from which
one can make conclusions on the 2nd order derivative for the atom 3 in
If I have the whole dynamical matrix I can diagonalize it and obtain
frequencies and eigenvectors.
If there are some errors please correct me.
Otherwise thanks again.
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