Force calculation and structure relaxation

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To calculate the atomic forces for a given structure without ion relaxation, set ‘calculation’ to ‘scf’, set input parameter ‘cal_force’ to 1.

calculation scf
cal_force 1

To relax the atom position without change cell shape, one needs to add a few more parameters in the INPUT file. Here is an example for the Si dimer. In this case, the forces are calculated by default.

calculation relax
gamma_only 1
relax_nmax 100
force_thr_ev 0.01
relax_method cg
out_stru 1
relax_bfgs_init 0.5

• calculation relax

  relax atom positions with fixed lattice vectors.

• relax_nmax

  the maximal number of ionic iteration steps.

• force_thr_ev

  the threshold for the force, below which the geometry relaxation is considered to be converged. The unit is eV/Angstrom.

• relax_method

  the algorithm used for geometry optimization. Possible choices are:

• cg

  conjugate gradient (CG) algorithm

• bfgs

  Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm

• cg_bfgs

  A mixed cg-bfgs method. For detail description, check out the variable relaxcgthr in the list of input keywords.

• sd

  steepest descend algorithm. The CG method is recommended.

• out_stru

  output the structure of each step or not.

• relax_bfgs_init

  the initial radius of the relaxation. We advise you not to change this parameter, unless you are sure that the initial structure is close to the final structure.

An example for structure relaxation calculation can be found at tests/108PWRE.

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