CELL_OPT
This section sets the environment for the optimization of the simulation cell. As is noted in FORCE_EVAL/SUBSYS/CELL, the program convention is that the first cell vector A lies along the X-axis and the second cell vector B is in the XY plane, such that the cell vector matrix is a lower triangle. There is no complete, official algorithm support and/or tests for updating the three upper triangular components during a cell optimization; please prepare input accordingly with these three components precisely 0 even for cases like the primitive rhombohedral cell of the FCC lattice. [Edit on GitHub]
Keywords
Keyword descriptions
- CONSTRAINT
Type: enum
Default: NONE
Usage: CONSTRAINT (none|x|y|z|xy|xz|yz)Description: Imposes a constraint on the pressure tensor by fixing the specified cell components.
Valid values:
NONEFix nothingXFix only x componentYFix only y componentZFix only z componentXYFix x and y componentXZFix x and z componentYZFix y and z component
Mentions: ⭐Geometry and cell optimization
- EPS_SYMMETRY
Type: real
Default: 1.00000000E-004
Usage: EPS_SYMMETRY {REAL}Description: 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.
- EXTERNAL_PRESSURE
Type: real[ ]
Default: 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 [bar]
Usage: EXTERNAL_PRESSURE {REAL} .. {REAL}Description: Specifies the external pressure (1 value or the full 9 components of the pressure tensor) applied during the cell optimization.
Mentions: ⭐Geometry and cell optimization
- KEEP_ANGLES
Type: logical
Default: F
Lone keyword: T
Usage: KEEP_ANGLES TRUEDescription: Keep angles between the cell vectors constant, but allow the lengths of the cell vectors to change independently during cell optimization. This is implemented by projecting out the components of angles in the cell gradient before the cell is updated. Albeit general, this is most useful for triclinic cells; to enforce higher symmetry, see KEEP_SYMMETRY.
Mentions: ⭐Geometry and cell optimization
- KEEP_SPACE_GROUP
Type: logical
Default: F
Lone keyword: T
Usage: KEEP_SPACE_GROUP .TRUE.Description: Detect space group of the system and preserve it during optimization. The space group symmetry is applied to coordinates, forces, the stress tensor, and the CELL_OPT cell metric. It works for supercell. It does not affect/reduce computational cost. Use EPS_SYMMETRY to adjust the detection threshold.
Mentions: ⭐Geometry and cell optimization
- KEEP_SYMMETRY
Type: logical
Default: F
Lone keyword: T
Usage: KEEP_SYMMETRY TRUEDescription: Keep the requested initial cell symmetry as specified in the FORCE_EVAL/SUBSYS/CELL section during cell optimization. This is implemented by removing symmetry-breaking components and taking averages of components if necessary in the cell gradient before the cell is updated. To enforce the space group (which requires spglib package), see KEEP_SPACE_GROUP.
Mentions: ⭐K-Points, ⭐Geometry and cell optimization
- KEEP_VOLUME
Type: logical
Default: F
Lone keyword: T
Usage: KEEP_VOLUME TRUEDescription: Keep the volume of the cell constant during cell optimization. This is implemented by comparing the cell volumes and scaling the new cell vectors just before updating the cell information, and can be used together with KEEP_ANGLES or KEEP_SYMMETRY.
Mentions: ⭐Geometry and cell optimization
- MAX_DR
Type: real
Default: 3.00000000E-003 [bohr]
Usage: MAX_DR {real}Description: Convergence criterion for the maximum geometry change between the current and the last optimizer iteration.
- MAX_FORCE
Type: real
Default: 4.50000000E-004 [bohr^-1*hartree]
Usage: MAX_FORCE {real}Description: Convergence criterion for the maximum force component of the current configuration.
- MAX_ITER
Type: integer
Default: 200
Usage: MAX_ITER {integer}Description: Specifies the maximum number of geometry optimization steps. One step might imply several force evaluations for the CG and LBFGS optimizers.
- OPTIMIZER
Type: enum
Default: BFGS
Aliases: MINIMIZER
Usage: OPTIMIZER {BFGS|LBFGS|CG}Description: Specify which method to use to perform a geometry optimization.
Valid values:
BFGSMost efficient minimizer, but only for ‘small’ systems, as it relies on diagonalization of a full Hessian matrixLBFGSLimited-memory variant of BFGS suitable for large systems. Not as well fine-tuned but can be more robust.CGconjugate gradients, robust minimizer (depending on the line search) also OK for large systems
References: Byrd1995
Mentions: ⭐Troubleshooting
- PRESSURE_TOLERANCE
Type: real
Default: 1.00000000E+002 [bar]
Usage: PRESSURE_TOLERANCE {REAL}Description: Specifies the Pressure tolerance (compared to the external pressure) to achieve during the cell optimization.
Mentions: ⭐Geometry and cell optimization
- RMS_DR
Type: real
Default: 1.50000000E-003 [bohr]
Usage: RMS_DR {real}Description: Convergence criterion for the root mean square (RMS) geometry change between the current and the last optimizer iteration.
- RMS_FORCE
Type: real
Default: 3.00000000E-004 [bohr^-1*hartree]
Usage: RMS_FORCE {real}Description: Convergence criterion for the root mean square (RMS) force of the current configuration.
- SHOW_SPACE_GROUP
Type: logical
Default: F
Lone keyword: T
Usage: SHOW_SPACE_GROUP .TRUE.Description: Detect and show space group of the system after optimization. It works for supercell. It does not affect/reduce computational cost. Use EPS_SYMMETRY to adjust the detection threshold.
- SPGR_PRINT_ATOMS
Type: logical
Default: F
Lone keyword: TDescription: Print equivalent atoms list for each space group symmetry operation.
- STEP_START_VAL
Type: integer
Default: 0
Usage: step_start_valDescription: The starting step value for the CELL_OPT module.
- SYMM_EXCLUDE_RANGE
Type: integer[2]
Repeatable: yes
Usage: SYMM_EXCLUDE_RANGE {Int} {Int}Description: Range of atoms to exclude from space group symmetry. These atoms are excluded from both identification and enforcement. This keyword can be repeated.
- SYMM_REDUCTION
Type: real[3]
Default: 0.00000000E+000 0.00000000E+000 0.00000000E+000
Usage: SYMM_REDUCTION 0.0 0.0 0.0Description: 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.