Dear @Lukas-jd,
To remove the negative frequency you can try two procedures.
Re-optimize the initial structure with a larger tolerance. You can make use of the EXTERNAL keyword using the f34 of the previous optimized structure.
For example:
EXTERNAL
OPTGEOM
FULLOPTG
TOLDEG
0.00010
TOLDEX
0.0004
TOLDEE
11
Run the SCAN-MODE to eliminate the negative frequency (see page 194 of Crysatl User's manual). To run the “scanmode” calculation:
select the mode you want to eliminate;
make an EXTERNAL, the f34 is the same used before;
select the initial, final, and the step for the scan;
run the calculation
Example for the input:
EXTERNAL
FREQCALC
SCANMODE
1 -10 10 0.4 (number of the mode to eliminate, first step, last step, step displacement)
2 (sequence number of the modes selected)
END
END
basis-set and etc
after finishing the scanmode calculation, open each SCAN_ file and plot a graphical of mode x energy (see example on user's manual).
vi. the minimal point should be your structure with minimal energy and no negative frequencies;
vii. run an optimization making an EXTERNAL from the SCAN_file (that is the f34) of the minimal energy found before;
viii. after, run the FREQCALC again.
Now, it should not appear negative frequency.
