To calculate the atomic forces for a given structure without ion relaxation, set ‘calculation’ to ‘scf’, set input parameter ‘force’ to 1.
calculation scf
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
nstep 100
force_thr_ev 0.01
move_method cg
out_stru 1
trust_radius_ini 0.5
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calculation
relaxrelax atom positions with fixed lattice vectors.
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nstep
the maximal number of ionic iteration steps.
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force_thr_ev
the threshold for the force, below which the geometry relaxation is considered to be converged. The unit is eV/Angstrom.
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move_method
the algorithm used for geometry optimization. Possible choices are:
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cg
conjugate gradient (CG) algorithm
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bfgs
Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm
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cg_bfgs
A mixed cg-bfgs method. For detail description, check out the variable cg_threshold in the list of input keywords.
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sd
steepest descend algorithm. The CG method is recommended.
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out_stru
output the structure of each step or not.
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trust_radius_ini
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/108_PW_RE.