Hi,

In CRYSTAL23, the anharmonicity of a given vibration mode - or of a set thereof - can be computed via: I) the evaluation of cubic and quartic interatomic force constants (in the basis of the normal modes) followed by II) the solution of the nuclear Schroedinger equation with either the vibrational self-consistent field (VSCF) or vibrational configuration interaction (VCI) method.

Details on the actual implementation of steps I) and II) can be found here:

I) Anharmonic force constants

II) VSCF and VCI for solids

Step-by-step, the procedure is as follows:

Geometry optimization to fully relax atomic positions within the cell (OPTGEOM keyword);
Harmonic frequency calculation (FREQCALC keyword);
Selection of the normal modes for which the anharmonic correction is to be computed (for instance, in your case, those corresponding to C-H stretching vibrations);
Calculation of cubic and quartic interatomic force constants for the selected modes + VSCF (or VCI) calculation of anharmonic states.

As an example, let' s assume that C-H stretching vibrations correspond to modes 15-20 in the list generated from the harmonic calculation (that is, there are 6 different C-H stretching modes). The input for steps 3. and 4. above would read:

FREQCALC
RESTART
ANHAPES
6
15 16 17 18 19 20
3 0.9
VSCF
END

that is, we restart the harmonic frequency calculation, and where 6 is the number of modes, 15 16 17 18 19 20 are the selected modes, and 3 0.9 are two parameters specifying the numerical approach used for the evaluation of cubic and quartic force constants.

Please, note that with such a calculation not only the "intrinsic" anharmonicity of each selected mode is evaluated but also the couplings among all selected modes. If, instead, one just wants to compute the "intrinsic" anharmonicity with no couplings, independent calculations can be run, one per each selected mode.

Visit this page for a tutorial on anharmonic calculations in CRYSTAL

Alessandro Erba