ORCA vibration
Table of Contents

Analytical Frequency


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:

DELQ 0.5

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.