frequency calculation in gaussian software

Use the Stable keyword to test the stability of Hartree-Fock and DFT wavefunctions. Note also that the CPHF (coupled perturbed SCF) method used in determining analytic frequencies is not physically meaningful if a lower energy wavefunction of the same spin multiplicity exists.

This may be accomplished automatically by specifying both Opt and Freq within the route section for a job. The recommended practice is to compute frequencies following a previous geometry optimization using the same method. It is also incorrect to compute frequencies for a correlated method using frozen core at a structure optimized with all electrons correlated, or vice-versa. This transformation is only valid at a stationary point! Thus, it is meaningless to compute frequencies at any geometry other than a stationary point for the method used for frequency determination.įor example, computing 6-311G(d) frequencies at a 6-31G(d) optimized geometry produces meaningless results. Vibrational frequencies are computed by determining the second derivatives of the energy with respect to the Cartesian nuclear coordinates and then transforming to mass-weighted coordinates. By default, the force constants are determined analytically if possible (for AM1, PM3, PM3MM, PM6, PDDG, RHF, UHF, MP2, CIS, all DFT methods and CASSCF), by single numerical differentiation for methods for which only first derivatives are available (MP3, MP4(SDQ), CID, CISD, CCD, CCSD, BD and QCISD), and by double numerical differentiation for those methods for which only energies are available. This calculation type keyword computes force constants and the resulting vibrational frequencies.

Frequency calculation in gaussian software