Harmonic Frequencies

Harmonic vibrational frequencies are computed from the nuclear Hessian (second derivative of the energy with respect to nuclear displacements). OpenQuantum supports both the fully analytical Hessian (default for RHF/UHF/RKS/UKS) and the semi-analytical finite-difference approach.

Harmonic Frequency Analysis — Water, RHF/STO-3G

task: FREQ RHF STO-3G

GEOMETRY
  O   0.000000   0.000000   0.117369
  H   0.756950   0.000000  -0.469476
  H  -0.756950   0.000000  -0.469476
END

FREQ
  Hessian     analytical
  Temperature 298.15
  Pressure    1.0
END

UHF Frequency Analysis with Analytical Hessian — OH Radical

task: FREQ UHF STO-3G

MOLECULE
  Multiplicity  2
END

GEOMETRY
  O   0.000000   0.000000   0.000000
  H   0.000000   0.000000   0.970000
END

Notes

  • The analytical Hessian is the default for RHF/UHF frequency calculations.
  • Use Hessian semi to force finite-difference of gradients.
  • For TS verification, the geometry must be a pre-converged TS structure.
  • A genuine TS shows exactly one imaginary frequency (negative value).

Read Hessian from Checkpoint

FREQ
  Read  myjob.oqd
END

This avoids recomputing the Hessian for subsequent thermochemistry at different temperatures/pressures.