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I am trying to relax (lattice parameters and atomic positions) the crystal structure of 2D system (slab model). I am able to get forces reduced with tags like cell_dofree= '2Dxy' or 'c', however pressure is too significant to accept. I am looking for suggestions on fully relaxing the 2D slab systems using quantum ESPRESSO. My vc-relax input file typically looks like :

calculation = 'vc-relax' etot_conv_thr = 4.0000000000d-05 forc_conv_thr = 1.0000000000d-04 outdir = './out/' prefix = 'NiTe' pseudo_dir = '../nc-sr-04_pbe_stringent' tprnfor = .true. tstress = .true. verbosity = 'low' nstep = 500 / &SYSTEM degauss = 2.0000000000d-02 ecutrho = 2.0000000000d+02 ecutwfc = 5.0000000000d+01 ibrav = 0 nat = 4 nosym = .true. ntyp = 2 occupations = 'smearing' smearing = 'cold' / &ELECTRONS conv_thr = 8.0000000000d-7 electron_maxstep = 80 mixing_beta = 4.0000000000d-01 / &IONS / &CELL cell_dofree = '2Dxy' / ATOMIC_SPECIES Ni 58.6934 Ni.upf Te 127.6 Te.upf ATOMIC_POSITIONS crystal Ni 0.6666666291 0.3333333415 0.4687894791 Te 0.6666667182 0.3333333228 0.5706946359 Ni 0.3333333703 0.6666666584 0.5312101766 Te 0.3333332823 0.6666666773 0.4293057084 K_POINTS automatic 20 20 1 0 0 0 CELL_PARAMETERS angstrom 3.8828468972 0.0000000000 0.0000000000 -1.9414234487 3.3626440520 0.0000000000 0.0000000000 0.0000000000 23.7475238445
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  • $\begingroup$ Why are you forcing the lattice parameters relaxation? $\endgroup$ Commented Nov 17, 2024 at 18:44
  • $\begingroup$ My aim is to construct Wannier functions from Bloch states. I am not sure whether I should do full relaxation or not. Kindly explain. $\endgroup$ Commented Nov 17, 2024 at 18:47
  • $\begingroup$ @Camps I am using CIF structure from MC2D database. Is it sufficient to relax atomic positions (i.e. minimise forces) only for 2D slab models? In other words, can we ignore residual non-negligible stress/pressure along all directions? $\endgroup$ Commented Nov 20, 2024 at 16:52

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Sometimes low ecut gives high pressure. Because of low quality. Did you do converge test?

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  • $\begingroup$ I am using norm-conserving pseudopotential for which ecutrho is recommeded to be 4 times ecutwfc. Should I increase? $\endgroup$ Commented Nov 19, 2024 at 6:00
  • $\begingroup$ In the case of norm-conserving pseudopotential, you just use default value(ecutrho = ecutwfc*4). I think you have to convergence test first. and take proper ecutwfc value. $\endgroup$ Commented Nov 19, 2024 at 6:06

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