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.
Vibrational frequencies.
Raman and IR activities and intensities (requires $rem DORAMAN).
Atomic masses.
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:
DORAMAN
Controls calculation of Raman intensities. Requires JOBTYPE to be set
to FREQ
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Do not calculate Raman intensities.
TRUE
Do calculate Raman intensities.
RECOMMENDATION:
None
VIBMAN_PRINT
Controls level of extra print out for vibrational analysis.
TYPE:
INTEGER
DEFAULT:
1
OPTIONS:
1
Standard full information print out.
If VCI is TRUE, overtones and combination bands are also printed.
3
Level 1 plus vibrational frequencies in atomic units.
4
Level 3 plus mass-weighted Hessian matrix, projected mass-weighted Hessian
matrix.
6
Level 4 plus vectors for translations and rotations projection matrix.
RECOMMENDATION:
Use the default.
CPSCF_NSEG
Controls the number of segments used to calculate the CPSCF equations.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
0
Do not solve the CPSCF equations in segments.
User-defined. Use segments when solving the CPSCF equations.
RECOMMENDATION:
Use the default.
Example 11.17 An EDF1/6-31+G* optimization, followed by a vibrational analysis. Doing the vibrational analysis at a stationary point is necessary for the results to be valid.
$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