CELL_OPT

This section sets the environment for the optimization of the simulation cell. Two possible schemes are available: (1) Zero temperature optimization; (2) Finite temperature optimization. [Edit on GitHub]

Subsections

Keywords

Keyword descriptions

CONSTRAINT: enum = NONE

Usage: CONSTRAINT (none|x|y|z|xy|xz|yz)

Valid values:

  • NONE Fix nothing

  • X Fix only x component

  • Y Fix only y component

  • Z Fix only z component

  • XY Fix x and y component

  • XZ Fix x and z component

  • YZ Fix y and z component

Imposes a constraint on the pressure tensor by fixing the specified cell components. [Edit on GitHub]

EPS_SYMMETRY: real = 1.00000000E-004

Usage: EPS_SYMMETRY {REAL}

Accuracy for space group determination. EPS_SYMMETRY is dimensionless. Roughly speaking, two scaled (fractional) atomic positions v1, v2 are considered identical if |v1 - v2| < EPS_SYMMETRY. [Edit on GitHub]

EXTERNAL_PRESSURE: real = 1.00000000E+002 0.00000000E+000 0.00000000E+000 0.00000000E+000 1.00000000E+002 0.00000000E+000 0.00000000E+000 0.00000000E+000 1.00000000E+002

Usage: EXTERNAL_PRESSURE {REAL} .. {REAL}

Specifies the external pressure (1 value or the full 9 components of the pressure tensor) applied during the cell optimization. [Edit on GitHub]

KEEP_ANGLES: logical = F

Lone keyword: T

Usage: KEEP_ANGLES TRUE

Keep angles between the cell vectors constant, but allow the lenghts of the cell vectors to change independently. Albeit general, this is most useful for triclinic cells, to enforce higher symmetry, see KEEP_SYMMETRY. [Edit on GitHub]

KEEP_SPACE_GROUP: logical = F

Lone keyword: T

Usage: KEEP_SPACE_GROUP .TRUE.

Detect space group of the system and preserve it during optimization. The space group symmetry is applied to coordinates, forces, and the stress tensor. It works for supercell. It does not affect/reduce computational cost. Use EPS_SYMMETRY to adjust the detection threshold. [Edit on GitHub]

KEEP_SYMMETRY: logical = F

Lone keyword: T

Usage: KEEP_SYMMETRY yes

Keep the requested initial cell symmetry (e.g. during a cell optimisation). The initial symmetry must be specified in the &CELL section. [Edit on GitHub]

MAX_DR: real = 3.00000000E-003

Usage: MAX_DR {real}

Convergence criterion for the maximum geometry change between the current and the last optimizer iteration. [Edit on GitHub]

MAX_FORCE: real = 4.50000000E-004

Usage: MAX_FORCE {real}

Convergence criterion for the maximum force component of the current configuration. [Edit on GitHub]

MAX_ITER: integer = 200

Usage: MAX_ITER {integer}

Specifies the maximum number of geometry optimization steps. One step might imply several force evaluations for the CG and LBFGS optimizers. [Edit on GitHub]

OPTIMIZER: enum = BFGS

Aliases: MINIMIZER

Usage: OPTIMIZER {BFGS|LBFGS|CG}

Valid values:

  • BFGS Most efficient minimizer, but only for ‘small’ systems, as it relies on diagonalization of a full Hessian matrix

  • LBFGS Limited-memory variant of BFGS suitable for large systems. Not as well fine-tuned but can be more robust.

  • CG conjugate gradients, robust minimizer (depending on the line search) also OK for large systems

References: BYRD1995

Specify which method to use to perform a geometry optimization. [Edit on GitHub]

PRESSURE_TOLERANCE: real = 1.00000000E+002

Usage: PRESSURE_TOLERANCE {REAL}

Specifies the Pressure tolerance (compared to the external pressure) to achieve during the cell optimization. [Edit on GitHub]

RMS_DR: real = 1.50000000E-003

Usage: RMS_DR {real}

Convergence criterion for the root mean square (RMS) geometry change between the current and the last optimizer iteration. [Edit on GitHub]

RMS_FORCE: real = 3.00000000E-004

Usage: RMS_FORCE {real}

Convergence criterion for the root mean square (RMS) force of the current configuration. [Edit on GitHub]

SPGR_PRINT_ATOMS: logical = F

Lone keyword: T

Print equivalent atoms list for each space group symmetry operation. [Edit on GitHub]

STEP_START_VAL: integer = 0

Usage: step_start_val

The starting step value for the CELL_OPT module. [Edit on GitHub]

SYMM_EXCLUDE_RANGE: integer[2]

Keyword can be repeated.

Usage: SYMM_EXCLUDE_RANGE {Int} {Int}

Range of atoms to exclude from space group symmetry. These atoms are excluded from both identification and enforcement. This keyword can be repeated. [Edit on GitHub]

SYMM_REDUCTION: real[3] = 0.00000000E+000 0.00000000E+000 0.00000000E+000

Usage: SYMM_REDUCTION 0.0 0.0 0.0

Direction of the external static electric field. Some symmetry operations are not compatible with the direction of an electric field. These operations are used when enforcing the space group. [Edit on GitHub]

TYPE: enum = DIRECT_CELL_OPT

Usage: TYPE (GEO_OPT|MD|DIRECT_CELL_OPT)

Valid values:

  • GEO_OPT Performs a geometry optimization (the GEO_OPT section must be defined) between cell optimization steps. The stress tensor is computed at the optimized geometry.

  • MD Performs a molecular dynamics run (the MD section needs must defined) for computing the stress tensor used for the cell optimization.

  • DIRECT_CELL_OPT Performs a geometry and cell optimization at the same time. The stress tensor is computed at every step

Specify which kind of method to use for the optimization of the simulation cell [Edit on GitHub]