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Controlling the relaxation process

If job relax or job relax_from_checkpoint or job relax_recycle_H is chosen, the variable relax_what allows different ways to relax a structure:

relax_what force

or

relax_what stress

or

relax_what force_and_stress

Default: relax_what force_and_stress

Another variable allows to manipulate the way the relaxation is done:

Set

relax_how gradient

to simply follow along the forces and stress, without updating the inverse of the Hessian matrix.

Default: update the Hessian

With the variable relax_how the line-minimization can be altered also:

relax_how slow        # do 3 steps per line minimization always
relax_how normal      # do up to 3 steps, but only if necessary
relax_how fast        # dont do line minimization at all

Default: relax_how normal

Note: This can be combined with the gradient option with a statement like relax_how normal_gradient

The parameter lambda_limit allows you to limit the size of relaxation steps to a certain number. Normally, lambda should be on the order of 1. Sometimes, especially in the beginning of the iteration, and if the H matrix is initialized to small, lambda can become to big, and the algorithm bombs out. It is then advisable to limit lambda with a line like that:

lambda_limit 4.0

Default: lambda_limit 1e6 (no limit)

The statement starting_distortion followed by nine floating point numbers allows to set an initial distortion of the lattice. This feature is only for debugging, and not recommended for use.


next up previous contents
Next: Pressure Up: Relaxing a structure Previous: Relaxing a structure   Contents
Jonathan Yates 2001-05-10