OPT Section

The OPT section controls geometry optimization, transition-state search, IRC, NEB, coordinate models, and constraints.

Keywords

Keyword	Type	Default	Description
`Algorithm`	`bfgs` \| `berny` \| `rberny` \| `sella` \| `geometric`	`berny`	Optimizer backend
`Coord`	`cartesian` \| `primitive` \| `dlc` \| `hdlc` \| `tric` \| `tric-p`	`cartesian`	Internal-coordinate model
`MaxCycle`	integer	100	Maximum optimization cycles
`Trust`	float	0.3	Initial trust radius (Bohr)
`TMax`	float	0.5	Maximum trust radius (Bohr)
`NoTrust`	boolean	false	Disable TRM level-shifted Newton step
`TransitionState`	boolean	false	Search for first-order saddle point
`IRC`	boolean	false	Intrinsic reaction coordinate follow
`IRCDir`	`+1` \| `-1`	both	Restrict IRC direction
`NEB`	boolean	false	Nudged elastic band
`Images`	integer	8	Number of NEB images
`SellaOrder`	`1` \| `2`	1	Sella gradient order (1 = Hessian-only)
`Step`	`trm` \| `prfo` \| `mmf`	`trm`	Step type (TRM, P-RFO, or MMF)
`Update`	`bfgs` \| `psb` \| `ms` \| `bofill` \| `sr1` \| `dfp` \| `bfgs_powell` \| `ts-bfgs`	`msp` / `ts-bfgs`	Hessian update formula
`Diis`	boolean	false	Enable GEDIIS
`DiisSize`	integer	6	GEDIIS subspace size
`Symmetry`	boolean	true	Exploit point-group symmetry in IC build
`MOGuess`	boolean	true	Reuse previous cycle's orbitals as SCF guess (warm start)
`OptSymmetry`	boolean	false	Enforce symmetry along the optimisation path
`NoStep`	boolean	false	Run optimizer without taking a step (debug)
`Hessian`	`analytical` \| `semi`	`analytical`	Hessian source for IC initialisation
`NoCartHessian`	boolean	false	Ignore any inline Cartesian Hessian
`CartHessian`	matrix	—	Inline Cartesian Hessian matrix
`Rigid`	boolean	false	Rigid-body component removal (TRIC only)
`RemoveTr`	boolean	false	Remove TR modes (DLC/HDLC/TRIC only)
`ConMethod`	`0` \| `1`	1	Delocalised construction method
`Connect`	boolean	false	Reconnect connectivity graph at every step
`AddCart`	boolean	false	Add Cartesian components to IC basis
`ConnectIsolated`	boolean	false	Connect isolated fragments
`Primitive`	boolean	false	Alias for `Coord primitive`
`PrimStep`	boolean	true	Step projection in primitive IC space
`Gauss`	`default` \| `loose` \| `tight`	`default`	Convergence preset
`Tighten`	integer	0	Tighten all criteria by 10⁻ᴺ
`MaxDisplacement`	float	0.5	Max allowed step length
`Constraints`	clause-list	—	Geometric constraints
`Dihedral`	boolean	`true`	(RBerny only) Include dihedral angles in the IC set
`SuperWeakDih`	boolean	`false`	(RBerny only) Use super-weak dihedral force constants
`EnergyNoise`	float	`2e-8`	(RBerny only) Energy-noise floor for trust-radius updates

Algorithm & Coordinate Model

`Algorithm`	Description
`bfgs`	Cartesian BFGS optimizer
`berny`	Berny RFO with internal coordinates
`rberny`	Rust Berny backend (native IC implementation)
`sella`	Saddle-point-aware IC optimizer (TS-BFGS)
`geometric`	GeomeTRIC native-TRIC quasi-Newton optimizer (RS-P-RFO for TS); aliases `geomet`, `tric`

`Coord`	Description
`cartesian`	Direct 3N optimisation
`primitive`	Full redundant primitive IC set
`dlc`	Non-redundant delocalised basis
`hdlc`	Hybrid DLC with Cartesian components
`tric`	TRIC (translation/rotation removed)
`tric-p`	TRIC with projected Cartesian components

| Update | Description | Note | |----------|-------------| | bfgs | Standard BFGS (minima) | working on | | psb | Powell–Symmetric–Broyden, rank-2 (TS) | working on | | ms | Murtagh–Sargent, rank-1 (TS) | working on | | bofill | φ·PSB + (1-φ)·MS, blends PSB and MS (TS) | working on | | sr1 | Symmetric rank-1 (naturally indefinite) | working on | | dfp | Davidon–Fletcher–Powell (PD) | working on | | bfgs_powell | Auto: BFGS for minima, Bofill for TS | working on | | ts-bfgs | TS-BFGS: BFGS in |H| space (saddle points) | working on |

Default: MSP for minima, TS-BFGS for TS (Sella), Bofill for TS (Berny).

Step Types

`Step`	Description
`trm`	Trust-region (level-shifted Newton)
`prfo`	Partitioned RFO eigenvector following (TS)
`mmf`	Minimum-mode following (flat surfaces, TS)

RBerny Backend Controls

When the optimizer backend is Algorithm rberny (or the RBerny task token), three additional backend-specific keywords are recognised in the OPT section. These fine-tune how the RBerny engine constructs its internal coordinates and manages the trust-radius update.

Dihedral

Controls whether dihedral (torsion) angles are included in the primitive internal-coordinate set. When set to false, dihedrals are omitted entirely, reducing the total coordinate count and removing torsional coupling from the Lindh Hessian guess.

OPT
  Algorithm  rberny
  Dihedral   false
END

SuperWeakDih

When enabled, dihedral angles that would normally receive a "weak" force constant (≈ 0.1 scaling factor in the Lindh Hessian guess) are instead treated as "superweak" (≈ 0.01 scaling factor). This gives very soft initial force constants for floppy torsional modes, preventing the first few optimisation steps from over‑correcting large‑amplitude dihedral motion.

OPT
  Algorithm     rberny
  SuperWeakDih  true
END

EnergyNoise

Estimated energy precision in Hartree. The trust‑radius update uses a noise floor of 10 × EnergyNoise: when the predicted energy change falls below this threshold the optimizer treats the change as numerically unreliable and adjusts the trust radius conservatively (neither growing nor shrinking it aggressively). Useful on flat potential‑energy surfaces where small energy differences can be dominated by numerical noise.

OPT
  Algorithm    rberny
  EnergyNoise  1e-7
END

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