Recommendations For The Calculation Of Harmonic Frequencies
analytic of numerical differentiation
If possible, use of analytic second derivatives is the prefered choice.
Large-scale calculations at CCSD and CCSD(T) level should be performed using partical AO algorithms (keyword: ABCDTYPE=AOBASIS); for closed-shell calculations in addition the module xecc should be used (keyword:CC_PROG=ECC).
Frozen-core orbitals (i.e., frozen inner-shell orbitals) can be used in all analytic second derivative calculations.
ROHF-MP and ROHF-CC calculations
No analytic second derivatives are currently available for correlated calculations based on ROHF reference functions.
UHF-CCSDT-n, UHF-CC3, and UHF-CCSDT calculations
No analytic second derivatives are currently available for CCSDT-n, CC3, and CCSDT calculations employing a UHF reference. Analytic second derivatives are also not available for QRHF-CC and B-CCD calculations.
Geometry in force-field calculations; rotational frequencies
Meaningful calculations of harmonic vibrational frequencies have to be carried out at an equilibrium geometry determined at the same quantum chemical level, at which the force-field calculation is to be carried out. The accuracy of the used equilibrium geometry can be checked via the magnitude of the rotational frequencies (printed in the CFOUR output) which should be not larger than a few wavenumbers.
Advantages of finite-difference calculations
Finite-difference calculations are recommended if, for example, only one symmetry block of the harmonic force constant matrix is required and if no analytic second derivatives are available. Finite difference calculations are thus currently the choice for computing harmonic vibrational frequencies at all ROHF-MP and ROHF-CC levels.
FCM, FCMINT, and DIPDER files
after a harmonic-force field calculation the file FCM contains the harmonic force constants in Cartesian coordinates, while the file FCMINT contains the corresponding force constants in an internal coordinate representation. File DIPDER contains the dipole derivatives in a Cartesian coordinate representation. Note that the files FCM and FCMINT might be used within a geometry optimization as Hessians.
Harmonic vibrational frequencies for non-standard isotopomers
can be obtained in two ways:
a) run a harmonic frequency calculation with a non-standard choice of masses via the %masses or isotopes' input
b) save after a frequency calculation for the main isotopomer the files JOBARC, JAINDX, OPTARC and supply a file ISOMASS (each line contains the appropriate mass for the corresponding atom in the ZMAT) or a file ISOTOPES (each line contains the appropriate isotope) and run xjoda. Note that this second procedure does not require a calculation of the whole force field and thus is usually preferred