Harmonic Vibrational Frequencies
Harmonic vibrational frequencies can be evaluated using analytic second derivative techniques at the following levels
Except for CCSDT-n, CC3, and CCSDT (closed-shell RHF only), this is possible for both RHF and UHF reference functions.
For CI methods as well as for CC methods with higher than triple excitations, analytic second derivatives are provided via the MRCC program by M. Kállay which has been interfaced to CFOUR (see Interface to MRCC).
Harmonic vibrational frequencies can be in addition computed by means of numerical differentiation of analytically evaluated gradients or via numerical differentiation of energies. The latter options are possible for all methods for which either energies and/or gradients are implemented in CFOUR.