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The default units for the geometry relaxation section are Ry for energies and bohr for distances.

Current constraints, important version notes:

- "
" keyword required to end this section, new to version 2.53`end geometry` - "
" keyword new, added in version 2.54`frame` - "
" keywords new, added in version 2.62`vgfixM`

The following are the common input keywords in this section.
All the keywords are optional, and can appear in any order within this section.
The keywords must be left-justified, but all data input is free-format.
`
`

**geometry relaxation**- begin relaxation input section- ...
**gsteps**- maximum number of geometry steps*n_step_g***ghistory**- max number of geometry steps to use in blend history*n_hist_g***no ges**- (*)turns off dynamic SCF guessing, reverts to overlapping spherical atom guess

(*) Moved to run phase data section (2.61)**gconv**- force convergence criterion (Ry/bohr)*force_conv***gblend**- initial update factor: R(2)=R(1)+gblend*F(1)*gblend***gmethod**- specify relaxation method*gmethod*(BROYDEN, ASD, DMDAT, DAMPED, STEEPEST, ...)**timestep**- time step for dynamical relaxation schemes*time_step***gfixed**- fix positions for sequence of atoms*about constraints**start_atom end_atom*(sequence to fix, inclusive)**grelax**- relax positions for sequence of atoms*about constraints**start_atom end_atom*(sequence to relax, inclusive)**frame**- constraint: fix atom I, vector I-J, plane I-J-K*about constraints**fixed_atom vec_atom plane_atom*-
**vgfixM**- axis-related constraints for atoms (new in 2.62)*about constraints* -
*num_fixed_atoms x_axis y_axis z_axis*(number of atoms bound by this constraint, and Cartesian axis)

*fixed_atom(1) fixed_atom(2) ... fixed_atom(num_fixed_atoms)*(list of atoms bound by this constraint)

- M = "a": free motion along axis, no motion normal to axis
- M = "d": fix axis-projected distance between listed atoms, free in-plane
- M = "p": no motion along axis vector, free in-plane

- ...
**dynamics**- invokes MD input section- ...
*MD data input* - end dynamics - end of MD input section
- ...
**end geometry relaxation**- end of relaxation input section

- fix selected atoms in place and allow the rest to totally relax

- fix the frame of reference defined by three atoms

- fix selected atoms using a single type of axis-based constraint It is not avaulable to apply more sophisticated constraints on the system. For example, it is

To select sequences of atoms to be relaxed/fixed-in-place, one can use:

- repeated invocations of
`gfixed` - repeated invocations of
`grelax`

Atom specification in these various constraints can either be by index within the list (e.g., "1 4" will select the first through fourth atom in a geometry list), or (as of 2.67) by the atom label within the list (e.g., "AT0001 AT0029" will select the atom sequence from the atom labeled "AT0001" through the atom labeled "AT0029", wherever those atom labels appear first in the list).) This allows for more flexible selections of atom sequences that do not required counting. Special names allow further flexibility in atom selection: "FIRST" and "LAST" will specify, naturally, the first atom, and last atom in the geometry list (e.g. "AT0002 LAST" will select the atoms from AT0002 through the last atom inclusive), provided that you have not already named an atom with "FIRST" or "LAST" (in which case the code will use the labeled atoms instead).

Another set of constraints is possible to impose on the atomic
configuration, given by the ** frame ** keyword.
The keyword requires three arguments:

The ** vgfixM** family of constraints allow for some interesting
and useful manipulations of the atom positions in a relaxation.
However, the implementation is ad hoc, requiring care in their use:

- A
`vgfixM`constraint must be specified after all other constraints in the input. - There can be only one invocation of a vgfixM constraint in an input.

** Important **:
The code does *not* check whether constraints
(** gfixed, grelax, frame, vgfixM **, etc.)
are compatible with the symmetry specified in the input.
If your constraints are incompatible with the symmetry (or with each other),
then when the code completes the first force calculation and attempts a geometry update,
it will stop the calculation when it discovers the (constrained) update violates symmetry.

- The geometry relaxation violated symmetry on the first update step.
- Check that constraints imposed on atomic relaxation are consistent with the symmetry.
- The geometry "blew up" on the first relaxation update step.
- Reduce: gblend, if doing Broyden; timestep, if doing a dynamical relax. The gblend/timestep factors for minimization usually work best when between 0.5 and 6.0, and the smaller these factors, the shorter the first jump.
- During Broyden, my relaxation "blew up" or violated symmetry.
- If you are not in a potential well, the second derivative matrix might be ill-behaved and Broyden will fail. Switch to the ASD method until in a basin (forces getting smaller and energy getting lower).
- Using ASD (or damped dynamics), the geometry update violated symmetry.
- Numerical noise - one atom of symmetrically related set of atoms got damped but others did not. Start geometry relaxation again with the last good geometry.
- Geometry relax will not converge (with Broyden, esp), and is in a well.
- Numerical noise - the convergence criterion might be smaller than the inaccuracy in the computed forces. Either increase force convergence criterion, or use a denser grid (to get more accurate forces).