ORCA vibration

Table of Contents

- SCF (HF, LDA, GGA, hybrid, ...)
- MP2
- ~~Double-Hybrid functional~~
- ~~meta-GGAs~~
- ~~RI-JK approximation~~
- ~~relativistic corrections~~

You can get results form a thermochemical analysis (based on ideal gas assumption) once `freq`

is turned on. The most useful quantity for me is the Gibbs free energy `G`

which includes contributions from electronic energies, vibrational frequencies and temperatures.

`G = H - T*S`

VPT2 module is available again since ORCA 5.0.0.
It is turned on by the keyword `NEARIR`

.
Since the evaluation of third and fourth derivatives are expensive, ORCA uses XTB2 to get high order derivatives by default.
The default settings are writen under `%FREQ`

:

%FREQ XTBVPT2 True DELQ 0.5 END

`DELQ`

is the step during the numerical differentiation in dimensionless normal mode units.
If one wants high order derivatives in the theory level as same as the single point calculation, `XTBVPT2 False`

can be used.
The total number of single point calculations in ORCA-VPT2 is `(2N_mode+1)(N_mode+1)`

where `N_mode = 3*N_atoms - 6`

for non-linear molecules.