Keywords Reference

All keywords are case-insensitive and use key = value syntax. Boolean values accept true/false.

Required Keywords

Keyword	Type	Description	Example
`program`	string	QM program to use	`gaussian`, `orca`
`method`	string	QM method and basis set	`B3LYP/6-31G*`
`nprocs`	integer	Number of processors	`4`
`mem`	string	Memory allocation	`4GB` (Gaussian), `8000` (ORCA MB/core)
`charge`	integer	Molecular charge	`0`
`mult_a`	integer	Spin multiplicity of state A	`1`
`mult_b`	integer	Spin multiplicity of state B	`3`

Gaussian — method goes in the route section:

method = n scf(maxcycle=500,xqc) uwb97xd/def2svpp scrf=(smd,solvent=acetonitrile)
mem    = 8GB

ORCA — method is the ! line (without the !):

method = B3LYP def2-SVP CPCM(2-octanone) VeryTightSCF
mem    = 8000

For ORCA, mem is the per-core memory in MB (%maxcore in the ORCA input).

Keyword	Type	Default	Description
`td_a`	string	`""`	TD-DFT keywords for state A
`td_b`	string	`""`	TD-DFT keywords for state B
`state_a`	integer	`0`	Excited state index for state A (0 = ground state)
`state_b`	integer	`0`	Excited state index for state B
`mp2`	boolean	`false`	Use MP2 gradients (Gaussian only)

td_state_a and td_state_b are accepted as aliases for td_a and td_b.

Keyword	Type	Default	Description
`mode`	string	`normal`	Run mode — see Run Modes
`max_steps`	integer	`100`	Maximum optimization steps
`max_step_size`	float	`0.1`	Maximum step size (Bohr)
`restart`	boolean	`false`	Restart from checkpoint file

Keyword	Type	Default	Description
`hessian`	string	`direct_psb`	Hessian update method (see Hessian Updates)
`use_gediis`	bool/string	`false`	DIIS variant: `false`=GDIIS, `true`/`sequential`=GEDIIS, `blend`=GDIIS_blend
`use_hybrid_gediis`	boolean	`false`	Enable sequential hybrid GDIIS/GEDIIS switching
`gediis_blend_mode`	string	`fixed_sequential`	Blend strategy: `fixed`, `fixed_sequential`, `gradient`, `sequential`
`gediis_switch_rms`	float	`0.005`	RMS gradient threshold for GDIIS → GEDIIS switch (Ha/Å)
`gediis_switch_step`	float	`0.001`	RMS displacement threshold for GEDIIS → GDIIS switch (Å)
`switch_step`	integer	`3`	Steps of BFGS before switching to DIIS
`max_history`	integer	`4`	DIIS history length
`smart_history`	boolean	`false`	Smart DIIS history (retains most useful points)
`reduced_factor`	float	`0.5`	Step size reduction factor for GDIIS
`step_reduction_multiplier`	float	`10.0`	RMS gradient multiplier for step reduction
`steepest_descent_step`	float	`0.01`	Fallback steepest-descent step size (Å)
`bfgs_rho`	float	`15.0`	BFGS step size scaling factor
`print_level`	integer	`1`	Verbosity: `0`=quiet, `1`=normal, `2`=verbose
`print_checkpoint`	boolean	`false`	Write checkpoint JSON at each step

Keyword	Type	Default	Description
`trust_reduction_factor`	float	`0.5`	Contraction factor when energy increases
`trust_increase_factor`	float	`1.2`	Expansion factor when energy decreases
`trust_inc_threshold`	float	`0.0001`	Energy increase threshold (Ha) for contraction
`trust_dec_threshold`	float	`0.0001`	Energy decrease threshold (Ha) for expansion
`trust_min_radius`	float	`0.01`	Minimum trust radius (Å)
`trust_max_radius`	float	`1.0`	Maximum trust radius (Å)

Keyword	Type	Default	Description
`delta_e`	float	`0.000050`	Energy difference threshold (hartree)
`rms_dis`	float	`0.0025`	RMS displacement threshold (Å)
`max_dis`	float	`0.004`	Maximum displacement threshold (Å)
`max_grad`	float	`0.001323`	Maximum gradient component threshold (Ha/Å)
`rms_grad`	float	`0.000945`	RMS gradient threshold (Ha/Å)

These options are only active when use_robust_diis = true.

Keyword	Type	Default	Description
`use_robust_diis`	boolean	`false`	Master switch for DIIS step validation
`gediis_variant`	string	`auto`	GEDIIS variant: `auto`, `rfo`, `energy`, `simultaneous`
`gdiis_cosine_check`	string	`standard`	Cosine threshold: `none`, `zero`, `standard`, `variable`, `strict`
`gdiis_coeff_check`	string	`regular`	Coefficient check: `none`, `regular`, `force_recent`, `combined`
`n_neg`	integer	`0`	Negative eigenvalues: `0`=minimum, `1`=TS search
`gediis_sim_switch`	float	`0.0025`	RMS error threshold for GEDIIS variant switching

See Robust DIIS for detailed descriptions.

`switch_step` value	Behavior
`3` (default)	BFGS for 3 steps, then DIIS
`0`	Pure DIIS from step 1 (fastest, needs good geometry)
`10`	Extended 10-step BFGS warm-up, then DIIS
`≥ max_steps`	Pure BFGS throughout (most stable)

Keyword	Type	Default	Description
`fixedatoms`	string	`""`	Fixed atom indices, e.g. `1,3-5,7`

Keyword	Type	Default	Description
`drive_type`	string	`""`	Coordinate type: `bond`, `angle`, `dihedral`
`drive_atoms`	string	`""`	Atom indices (comma-separated)
`drive_start`	float	`0.0`	Starting coordinate value
`drive_end`	float	`0.0`	Ending coordinate value
`drive_steps`	integer	`10`	Number of drive steps
`drive_coordinate`	string	`""`	Reaction coordinate specification

Keyword	Type	Default	Description
`gau_comm`	string	`g16`	Gaussian executable name or path
`orca_comm`	string	`orca`	ORCA executable name or path

Keyword	Type	Default	Description
`isoniom`	boolean	`false`	Enable ONIOM multi-layer calculation
`chargeandmultforoniom1`	string	`""`	Charge/multiplicity specification for state A
`chargeandmultforoniom2`	string	`""`	Charge/multiplicity specification for state B

Keyword	Type	Default	Description
`charge_b`	integer	—	Separate charge for state B (overrides `charge`)
`basis`	string	`""`	Basis set specification
`solvent`	string	`""`	Solvent model
`dispersion`	string	`""`	Dispersion correction
`custom_interface_file`	string	`""`	Path to custom QM interface JSON config
`fix_de`	float	`0.0`	Target energy difference for fix-dE mode (eV) (temporarily disabled)