Warning about frequencies with COSMO model

Numerical frequencies calculated with COSMO should be checked for stability with respect to the disrad, the numerical differentiation step size. The problem is that the COSMO surface changes slightly when a nucleus is moved from its equilibrium position. The change is usually smal but in some cases it may result is creation or annihilation of surface points, which will lead to discontinuities in the potential energy surface and may result in inaccurate frequencies.

Thus, when calculating vibrational frequencies numerically with COSMO, one should try decreasing the disrad value until no changes in frequencies are observed. However, the value should not be too small because then the total numerical noise may become too large compared to the generated forces. A general recommendation would be to try to decrease disrad by a factor of 2 at a time. Of course, this procedure may be very expensive for a large molecule. If this the case, one should use the SCANFREQ keyword and recalculate only a small number of frequencies. It should be noted that generally frequencies that have a small force constant are more sensitive to the numerical noise.