Question 1
I want to calculate the zero point energy for my system in order to compare my theoretical results with experimental data.Firstly, I have performed the geometry optimization calculations for my system. And secondly, I want to calculate the normal mode of my system. I wonder if my INCAR listed below is right, and if this kind of calculation will change my geometry again.
Thanks a lot!
Start parameter for this Run: ISTART = 0 ICHARG = 2 INIWAV = 1 Electronic Relaxation: ENCUT = 350 ENAUG = 550 PREC = Accurate IALGO = 38 NELM = 60 NELMIN = 4 NELMDL = -5 EDIFF = 1E-05 NBANDS = 256 GGA = 91 VOSKOWN = 1 Ionic Relaxation: EDIFFG = 1E-04
NSW = 1 -------------------> Do NSW=1 and ISIF =2 make me optimize the geometry again? IBRION = 5 ISIF = 2 NFREE = 2
DOS related values: SIGMA = 0.1 ISMEAR = 1 Spin polarized: ISPIN = 2
no, if IBRION=5, no further geometry optimisation step is taken, the displacements of the atoms are just to determine the Hessian matrix.
Please do not forget to set POTIM to a small value which ensures that the displacements are in the harmonic regime
Question 2: Hello Everyone,
I am wondering how to calculate the zero point energy ? Or is it included in the total energy EO in the OUTCAR file ? Thank you
For a periodic system you need to calculate the phonon density of states. See the discussion here: http://cms.mpi.univie.ac.at/vasp-forum/forum_viewtopic.php?4.1543
For an isolated molecule you need to do the vibrational analysis, just like any other quantum chemistry approach.
Qustion 3:
I would like to produce a phonon dispersion relation. Can someone give me some hints where to start how to proceed from the VASP generated output.
So far I have understood following:
1. IBRION=5 in INCAR is needed, together with ISYM =0
POTIM=0.03 (for example) and NSW=1.
2. For accurate results I need a large supercell and many k-points, although I don't yet really understand why.
3. A third party software, called \ phonon dispersion relation with VASP, although this was with older versions of VASP. Can VASP version 4.6 do this without extra software?
Thank you, Rob.
You need a large supercell because the \that fit inside the supercell. These wavelengths of course vary a lot with the material. For the crystals I deal with (complex metal hydrides) cells of edge length of the order of 10 Ang are sufficiently large.
You need accurate forces. I'm not convinced that you necessarily need many k-points, but then I usually converge my energies w.r.t. cutoff and k-points to sub-meV levels.
IBRION=5 will allow you to calculate phonons at the gamma point only. There are standard integrals to go from these to the dispersion relations. (See, for example, Maradudin et al., \of lattice dynamics in the harmonic approximation\in Solid State Physics Supplement 3, Academic Press, New York, 1971.)
Phonon, available (at a price) either as part of the Materials Design suite or directly as a binary from Krzysztof Parlinski (http://wolf.ifj.edu.pl/phonon/), does the full k-space integration for accurate properties.
About the requirement of a large supercell, what I understand is that large supercells better
reproduce the interactions(particularly long range) than their smaller counterparts. I do not understand how it is related to wavelength of phonon without much elaboration possibly with some refs.
You could try Parlinski, Li and Kawazoe, PRL 78 (1997) 4063. Though in my opinion it's not expressed very well there. The Phonon manual is better.
The phonons are only accurate for all wavevectors within your zone if the interactions with regions outside your supercell are negligible.
Thank you 'tjf' and 'bandy' for your comments.
I still have some basic questions:
1) When I use in my INCAR file NSW = 1 IBRION = 5 POTIM = 0.05 NFREE = 2 ISYM = 0
then is the corresponding output only in the OUTCAR file, or is there more elsewhere?
2) I don't fully understand the related output in OUTCAR, neither is it explained in the VASP guide. The relevant parts of the OUTCAR, which I mostly do not understand, I have put here: http://surfion.snu.ac.kr/~lahaye/vasp/
Also the corresponding INCAR and POSCAR files are there.
For example, what do the numbers in the DYNMAT header mean: DYNMAT ------
1 32 3 12.011
1 1 0.0500 0.0000 0. 0000
which corresponds to ? number-of-atoms ? mass-of-C-atom ? ? POTIM ? ? Thanks, Rob.
the output lines are DYNMAT ------
number of atom types (1) , number of atoms (of each type) (32), number of degrees of freedom (3) atomic mass (es for each type)
#of the degree of freedom (1), #of the displacement (1) , displacement stepwidth (x,y,z) (POTIM 0.0 0.0)
admin wrote: ... the output lines are DYNMAT ------
number of atom types (1) , number of atoms (of each type) (32), number of degrees of freedom (3) atomic mass (es for each type)
#of the degree of freedom (1), #of the displacement (1) , displacement stepwidth (x,y,z) (POTIM 0.0 0.0)
and the rest of the lines? They differ from the TOTAL-FORCES given in the OUTCAR file...
Question 4:
Finally \and error\i found the rigt SMASS>0 value for my bulk system. now temperature ossilations behave nicely around the specified tempereture. For those who have trouble with crazy temperature ossilations in Nose-Hover, i recommend to try SMASS values on a small system with