Vibrational Frequency Computations

PSI3 is also capable of computing harmonic vibrational frequencies for a number of different methods using energy points or analytic energy first or second derivatives. (At present, only RHF-SCF analytic second derivatives are available.) If analytic energy second derivatives are not available, PSI3 will generate displaced geometries along symmetry adapted cartesian coordinates, compute the appropriate energies or first derivatives, and use finite-difference methods to compute the Hessian.

The following keywords are pertinent for vibrational frequency analyses:

JOBTYPE = string

This keyword must be set to FREQ for frequency analyses.

DERTYPE = string

This keyword may be set to NONE if only energies are available for the chosen method, or FIRST if analytic gradients are available.

POINTS = 3 or 5

Specifies whether frequencies are determined by a 3-point or a 5-point formula of gradient differences. If only energy points are used, more displacements are required, but the effect of this keyword in terms of accuracy is the same.

Note: In some situations, vibrational frequency analysis via finite differences may fail if the full point group symmetry is specified via the symmetry keyword. This happens because the user-given symmetry value can become incompatible with the actual symmetry of the molecule when energies or gradients are evaluated for symmetry-lowering displacements. In such situations, the user is advised to let the program determine the symmetry automatically, rather than specifying symmetry manually. Otherwise, an error such as the following may result:

error: problem assigning number of operations per class
         *** stopping execution ***

The manual pages for the normco and intder95 modules contain information on additional tools useful in vibrational frequency analysis and coordinate transformation.