Crystal: Optimization

Lukas.jd

I’m trying to optimize a single-wall carbon nanotube,SWCNT, but it shows an imaginary frequency (-885.4033 cm-1). Thus, this structure corresponds to saddle point. How I can remove this negative frequency to find a true minima?

CrystalSupport

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.

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Crystal: Optimization