* analytic of numerical differentiation*

If possible, use of **analytic second derivatives** is the prefered choice.

*Large-scale calculations*

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 calculations*

**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

*example for a frequency calculation using non-standard isotopes*