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I am using VASP to perform a structural relaxation of $\ce{CuNCN}$. I am using the PAW-PBE POTCAR files for the same and the KPOINTS have been generated using a script. The POSCAR file has been generated from the CIF file obtained via PXRD measurements. Even after multiple iterations (using the CONTCAR as POSCAR in subsequent calculations), the CONTCAR data is nowhere close to my POSCAR data, i.e. the structure is relaxing to an incorrect energy minimum. Note that I am using the DFT-D3 dispersion scheme for the optimization. In the literature, dispersion has been employed for $\ce{PbNCN}$, which has a similar layered structure.

My POSCAR structure: enter image description here

My CONTCAR structure (After 5 runs): enter image description here

My INCAR:

ISTART = 0
ICHARG = 2
PREC = Accurate
ISIF = 3
IBRION = 2
EDIFF = 0.00000001
EDIFFG = -0.0005
NSW = 200
NELM = 100
ENCUT = 400
PSTRESS = 0
IVDW = 12
ISMEAR = 0
SIGMA = 0.1
ISYM = 1
LMAXMIX = 4
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This still looks okay to me, the bonds you see in VESTA are not real in the sense that they don't mean a covalent bond exists. There may be some weak bonding between layers (Are you certain this material forms this structure exact or is it something you propose?). The bond lengths between layers look very long, so I would not overthink it.

The only other option I can think of is to check if other functionals produce the same "incorrect" structure at the end. I don't see you specifying a particular functional, so it will default to LDA. PBE and PBEsol might be good checks to see if this happens still.

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  • $\begingroup$ Agree with this answer. Note that Materials Project does have the layered structure, and this is not using dispersion corrections at all and only using PBE, see materialsproject.org/materials/mp-3421 -- so I think this is probably a visualization artifact from VESTA. $\endgroup$ Jun 23 at 18:51
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Two structures of POSCAR and CONTCAR you show are the same. When you open VESTA, you choose Edit --> Bond, then you edit a same Max. length. VESTA would show the same.

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