[QE-users] Monoclinic structure optimization

[Luiz Gustavo Davanse da Silveira]

Dear QE users,

I am trying to optimize NiTi (nitinol) martensite structure (monoclinic,
space group P21/m) via a vc-relax calculation with cell_dofree = 'ibrav'
option. The problem is that QE gives me a gamma angle of almost 90° while
the experimental value is around 97°. I tried to construct an energy vs
gamma curve keeping the angle fixed and relaxing the axis with cell_dofree
= 'volume' but it doesn't work since the axis are not orthogonal. I don't
know what else I can try. If anyone could give me a hint it will be very
much appreciated.

Find below my input file and the final coordinates outputed by QE.

&CONTROL
        calculation = 'vc-relax'
        outdir = './outdir'
        prefix = 'niti'
        etot_conv_thr = 1.0E-4
        pseudo_dir = './pseudo'
        disk_io = 'none'
/
&SYSTEM
        ibrav = 12
        A = 2.8
        B = 4.5
        C = 4
        cosAB = -0.1184
        nat = 4
        ntyp = 2
        ecutwfc = 110
        ecutrho = 1760
        occupations = 'smearing'
        degauss = 0.06
        smearing = 'gauss'
/
&ELECTRONS
        conv_thr = 1.E-8
/
&IONS
/
&CELL
        cell_dofree = 'ibrav'
/
ATOMIC_SPECIES
Ni 58.6934 Ni.pbesol-spn-kjpaw_psl.1.0.0.UPF
Ti 47.867 Ti.pbesol-spn-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS {crystal}
Ni 0.9475 0.8070 0.25
Ni 0.0525 0.1930 0.75
Ti 0.5274 0.2790 0.25
Ti 0.4726 0.7210 0.75
K_POINTS {automatic}
14 9 10 0 0 0



CELL_PARAMETERS (alat=  5.29123317)
   1.002403137   0.000000000   0.000000000
  -0.005714828   1.587445555   0.000000000
   0.000000000   0.000000000   1.513222861

ATOMIC_POSITIONS (crystal)
Ni            0.9995289657        0.8063481974        0.2500000000
Ni            0.0004710343        0.1936518026        0.7500000000
Ti            0.5020451582        0.2794391874        0.2500000000
Ti            0.4979548418        0.7205608126        0.7500000000



Best regards,
Luiz Gustavo D. Silveira
UFPR - Brazil