Initialization of the inverse of the Hessian matrix

The Broyden scheme used for the relaxation of the forces and the stress updates an inverse of the Hessian matrix, which essentially describes the enthalpy parabola around the minimum. There are some open parameters which determine the values to which the matrix is initialized. They basically determine the length of the trial steps during the first few iterations:

relax_eps_factor   0.1     initialization perfactor for the stress part
of H
relax_coord_factor 0.1     initialization perfactor for the force part
of H

Try with the values above for the first run. Adjust later such that the first few steps give a lambda of about 0.5 to 1.

Default:

relax_coord_factor 1.0
relax_eps_factor   assuming the bulk modulus is about 1 Mbar

Alternatively, you can specify the physical quantities to which the above two variables correspond. This is the recommended way:

estimated_bulk_modulus  1.0                 Bulk modulus in MBar
estimated_optical_phonon_frequency 12.0     TO phonon frequency f at
                                            Gamma in THz)

Diamond would have a bulk modulus of about 4.4 MBar and around 35 THz for the optical phonon frequency.

For molecular solids, the option

decouple_coordinates yes

causes a different initialization of the Hessian matrix, which decouples the coordinates of the molecule from the lattice vectors to first order. This means that if the lattice vectors change 10 percent, the cartesian coordinates will change by about 1 percent. IF THIS OPTION IS USED, THE SYMMETRY OF THE CRYSTAL IS NOT PRESERVED!.

Default:

decouple_coordinates no