FREQ Section

The FREQ section controls harmonic frequency analysis and thermochemistry.

Keywords

Keyword	Type	Default	Description
`Hessian`	`analytical` \| `semi`	`analytical`	Hessian source (default: analytical for RHF/UHF)
`Numerical`	boolean	false	Force finite-difference Hessian
`Step`	float	0.01	Finite-difference step (Bohr)
`Temperature`	float	298.15	Temperature for thermochemistry (K)
`Pressure`	float	1.0	Pressure for thermochemistry (atm)
`Scale`	float	1.0	Frequency scaling factor for ZPE / thermal corrections
`SymmetryNumber`	integer	auto	Rotational symmetry number (auto-detected)
`Read`	file	—	Read Hessian from checkpoint file
`Save`	boolean	false	Write Hessian to checkpoint after computation

Standard frequency analysis:

FREQ
  Hessian     analytical
  Temperature 298.15
  Pressure    1.0
END

Force numerical Hessian:

FREQ
  Numerical true
  Step        0.01
END

Read Hessian from checkpoint:

FREQ
  Read  myjob.oqd
END

Custom thermochemistry conditions:

FREQ
  Temperature 350.0
  Pressure    1.0
  Scale       0.9854
END

Custom symmetry number:

FREQ
  SymmetryNumber 2
END

The analytical Hessian is the default for RHF/UHF single-point frequency calculations.
Hessian semi forces finite-difference of gradients (slower but can be used for methods without analytical Hessian).
The symmetry number is auto-detected from the point group. Override with SymmetryNumber N if needed.
Frequency scaling factor applies to ZPE and all thermal corrections.
Temperature and pressure affect thermal corrections and entropy.