`The 'difference between energy and force approaches' is just that: a`

`different approach to find the equilibrium positions in space for the`

`atoms and their electrons within the structural model given by the`

`.struct file.`

`The equilibrium is the energy minimum in the parameter space under`

`consideration. An energy minimum means that the derivatives with respect`

`to (atom) positions - that is the forces - vanish. Knowing the`

`derivatives (forces) helps, of course, a lot in finding the minimum -`

`they point the way to the next (local) minimum.`

In the user guide you see: ------User guide ------ 5.3.1 Lattice parameters (Volume, c/a, lattice parameters) Package optimize

`The auxilliary program optimize (x optimize) generates from an existing`

`case.struct (or`

`case initial.struct, which is generated at the first call of optimize) a`

`series of struct files`

`with various volumes (or c/a ratios, or other modified parameters)`

`(depending on your input):`

. . .

`After execution of this script you should have a series of scf-files`

`with energies corresponding to the`

`modified parameters, which should allow you to find the corresponding`

`equillibrium parameters.`

`For the volume optimization an analysis tool is available, other tools`

`are under development).`

----------------------

`The minimum total energy defines the equilibrium. The derivatives with`

`respect to lattice parameters are not easy to obtain during the scf`

`cycle so for lattice parameters an 'energy approach' is used.`

`For the internal parameters this is different. Persons as ingenious as`

`Prof. Marks can calculate the derivatives with respect to internal`

`parameters from the charge distribution at affordable computational cost`

`during the scf.`

------User guide ------ 5.3.2 Minimization of internal parameters (min lapw)

`Most of the more complicated structures have free internal structural`

`parameters, which can either`

`be taken from experiment or optimized using the calculated forces on the`

`nuclei.`

`Starting with WIEN2k 11.1 there are two possibilities to determine the`

`equilibrium position of all`

`individual atoms automatically (obeying the symmetry constraints of a`

`certain space group). One`

can use either

the shell script min lapw, together with the program mini, which willrun a scf-cycle, update

`the positions using the calculated forces and restarts a new scf cycle.`

`This continues until`

forces drop below a certain value;

or use the normal scf-scripts run lapw -min where in case.inm theswitch MSR1 will be

`modified to MSR1a such that the charge density and the positions are`

`simultaneously opti-`

mized during the scf-cycle. ------------------------------

`The first option uses what you call the 'force approach': in`

`equilibrium, no forces should push the atoms around.`

`The second option indicates a mixed approach: The positions of the atoms`

`(according to forces acting on them) and the ones of the electrons (to`

`minimize the total energy) are BOTH adjusted in each step of the scf.`

`As I said, there can be many reasons why your calculation did not reach`

`convergence for some structural parameters. Did the scf stop without`

`errors because the maximum number of iterations was reached? If yes,`

`what was this number of iterations? Maybe your convergence criteria are`

`too strong for the numerical precision you set by parameters like RKMAX`

`or the k-mesh? Maybe the scf oscillates between several good solutions`

`(is it a magnetic case?). Maybe your starting configuration and/or the`

`model Hamiltonian is completely off or missing some ingredient and the`

`poor scf wanders helpless around, lost in a multidimensional world ...`

Best regards, Martin Pieper --- Dr. Martin Pieper Karl-Franzens University Institute of Physics Universitätsplatz 5 A-8010 Graz Austria Tel.: +43-(0)316-380-8564 Am 13.11.2016 19:14, schrieb Abderrahmane Reggad:

Thank you Dr Pieper for your interesting to my questions. I have optimized the atomic positions before doing calculation. Tha thing that I didn't understand is that the convergence is reached for some points but not for others. For the "optimization notes " , there is no mention on the difference betwwen the energy and force approaches. Best regards -- Mr: A.Reggad Laboratoire de Génie Physique Université Ibn Khaldoun - Tiaret Algerie _______________________________________________ Wien mailing list Wien@zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html

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