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Hi folks,

We're using VASP to refine the inulin unit cell (SG: P212121) with 240 C, H and O atoms in total in the POSCAR file. The original a,b,c taken from simulation of its X-ray powder diffraction data and the literature is 17.480, 9.657, 14.162, but after VASP modelling we have 15.341, 8.891 and 13.145.

Does anybody have an idea why we had such a big change? We know VASP assumes zero Kelvin for its calculation, but is this the reason for that huge difference? We're using 250 eV energy cut-off and a Monkhorst-pack K point mesh of 1*1*1 for the calculation. We understand that ideally we should set K point to e.g. 3*2*3 for the VASP calculation, but that would take forever as the inulin unit cell is massive!!

If anybody has done VASP for a sugar type unit cell before (or knows what is going on here) and can offer us some suggestions, that would be very much appreciated!

Cheers
Andrea & Gujie

PS. We also put IVDW=12 and ISYM=2 tags into the INCAR file.

Hi there,

your cut-off energy is far to small for energy and cell optimisation! Two reasons:
- With 250 eV you have(!) to choose O_s, where the core radius is too big for C-O and O-H bonds. So the program adjusts your 'too short bond' and heureka, your cell becomes too large. Check your output for that, please!
- Use a gamma-centred K-point mesh. If I'm not mistaken with your molecule (e.g. https://en.wikipedia.org/wiki/Inulin), there will be NO harm in reducing effort at this side. Your cell is large enough as well as the system is non-metallic. You still can check for changes with a well converged structure. Start at the cheap end. Use the proper VASP version (gamma point only) and you'll save some more computer time.
- Advice:
* Use standard pseudopotentials as far as available, e.g. named C, O, H. Bond lengths are fine with that.
* Use around 800 eV cutoff energy. I know it sounds sick, but this is the way to overcome Pulay stress.
* Take a look at possible van der Waals interactions. It might mess up your cell size, too!
* and most importantly:

Have fun! :-)

alex

Many thanks for your reply and suggestions, Alex!!