In order to carry out a frequency analysis users must at a minimum provide a molecule within the $molecule keyword and define an appropriate level of theory within the $rem keyword using the $rem variables EXCHANGE, CORRELATION (if required) (Chapter 4) and BASIS (Chapter 8). Since the default type of job (JOBTYPE) is a single point energy (SP) calculation, the JOBTYPE $rem variable must be set to FREQ.
It is very important to note that a vibrational frequency analysis must be performed at a stationary point on the potential surface that has been optimized at the same level of theory. Therefore a vibrational frequency analysis most naturally follows a geometry optimization in the same input deck, where the molecular geometry is obtained (see examples).
Users should also be aware that the quality of the quadrature grid used in DFT calculations is more important when calculating second derivatives. The default grid for some atoms has changed in Q-Chem 3.0 (see Section 5.5) and for this reason vibrational frequencies may vary slightly form previous versions. It is recommended that a grid larger than the default grid is used when performing frequency calculations.
The standard output from a frequency analysis includes the following.
Raman and IR activities and intensities (requires $rem DORAMAN).
Zero-point vibrational energy.
Translational, rotational, and vibrational, entropies and enthalpies.
Several other $rem variables are available that control the vibrational frequency analysis. In detail, they are:
$molecule 0 1 O C 1 co F 2 fc 1 fco H 2 hc 1 hco 3 180.0 co = 1.2 fc = 1.4 hc = 1.0 fco = 120.0 hco = 120.0 $end $rem JOBTYPE opt METHOD edf1 BASIS 6-31+G* $end @@@ $molecule read $end $rem JOBTYPE freq METHOD edf1 BASIS 6-31+G* $end