Frequencies#

The Frequencies sub-step computes the analytic Hessian and reports harmonic vibrational frequencies, IR intensities, and thermochemistry quantities. By default it first optimizes the geometry (xTB’s --ohess workflow), since Hessians on non-stationary geometries are not physically meaningful.

Dialog#

The dialog has three panels.

Hamiltonian Parameters#

The same panel as in Energy and Optimization.

Optimization#

The same panel as in Optimization. The optimization-level and structure-handling settings are used when Optimize first is set to yes; they are ignored when it is set to no.

Frequencies / Thermochemistry#

Optimize first

Whether to optimize the geometry before computing the Hessian.

  • yes (default) – uses --ohess, the recommended xTB workflow.

  • no – uses --hess and assumes the input geometry is already a stationary point. Use this only if you have just run an Optimization sub-step or are confident the geometry is converged tightly enough.

Temperature

Temperature for the thermochemistry table. Defaults to 298.15 K (standard conditions). Note that v1 of the plug-in only uses xTB’s default temperature; non-default values will be supported in a future release via xTB’s xcontrol file. If you request a non-default temperature, a warning is printed.

Pressure

Pressure for the thermochemistry table. Defaults to 1 atm. xTB’s thermo treatment uses the ideal-gas approximation; pressure enters only through the standard-state correction.

Output#

The results table augments the energy table with frequency-count and thermochemistry rows:

        xTB (GFN2-xTB) Frequencies / Thermochemistry
╭─────────────────────────────────────────────────────┬─────────────┬────────────╮
│                      Property                       │      Value  │   Units    │
├─────────────────────────────────────────────────────┼─────────────┼────────────┤
│                Number of frequencies                │      9      │            │
│                Imaginary frequencies                │      0      │            │
│                  The total energy                   │   -5.070544 │   E_h      │
│ The electronic energy (excluding nuclear repulsion) │   -5.096007 │   E_h      │
│                  The HOMO-LUMO gap                  │   14.3730   │   eV       │
│        The molecular dipole moment magnitude        │    2.2109   │   debye    │
│          The zero-point vibrational energy          │   52.78     │   kJ/mol   │
│              The thermal enthalpy H(T)              │   62.71     │   kJ/mol   │
│            The entropic contribution T*S            │   56.18     │   kJ/mol   │
│                    The entropy S                    │  188.4      │  J/mol/K   │
│             The Gibbs free energy G(T)              │    6.52     │   kJ/mol   │
│    The total free energy (electronic + G(RRHO))     │ -13311.78   │   kJ/mol   │
│       The temperature for the thermochemistry       │  298.15     │   K        │
╰─────────────────────────────────────────────────────┴─────────────┴────────────╯

Frequencies, IR intensities, and reduced masses are also stored as arrays in the property database (one entry per mode).

Number of imaginary frequencies#

For a true minimum, Imaginary frequencies should be 0. For a genuine transition state it should be 1. Larger values indicate the geometry is on a higher-order saddle or that the optimizer failed to converge tightly enough – tighten the optimization level and rerun.

Note that xTB conventionally reports imaginary frequencies as negative values in its output (i.e. -150 cm-1 instead of 150i). The Imaginary frequencies count here is just the number of entries with a negative value.

The work directory contains, in addition to the Energy and Optimization files, vibspectrum (Turbomole-format IR spectrum), hessian (the Hessian matrix), and g98.out (Gaussian-98-format file readable by many visualization tools).