Harmonic and Anharmonic Lattice Dynamics and Thermodynamics

Hi,

I think that the best way to proceed in this case would be to restart the calculation one or more times. To do this, you just have to make sure that the scratch folder is not deleted when the calculation stops. Basically, you would run a first job with:

FREQCALC END

Then, after the first calculation stops (maybe because you reached a wall clock time limit) you would run a second job restarting from the first as:

FREQCALC RESTART END

In order to make this restart work, you need a few files from the previous job to be placed in the scratch folder of the new job: FREQINFO.DAT, fort.13 and fort.9 (to be renamed fort.20 in the new folder).

If needed, you can repeat this restart process multiple times until completion of the frequency calculation.

Hope this helps,

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