Hello.

This is very odd. Please attach your output file after relaxation process is done, then maybe something can be said.

A Little bit long, but the whole output.... giving in a Visualization Programm the same atomic positions as in the Input. But it should went to substantially different rhomboedric Phase at 0 K.

Program PWSCF v.5.4.0 starts on 11Aug2016 at 10:43:16

     This program is part of the open-source Quantum ESPRESSO suite

     for quantum simulation of materials; please cite

         "P. Giannozzi et al., J. Phys.:Condens. Matter 21 395502 (2009);

          URL http://www.quantum-espresso.org",

     in publications or presentations arising from this work. More details at

     http://www.quantum-espresso.org/quote

     Serial version

     Waiting for input...

     Reading input from standard input

     Message from routine read_cards :

     DEPRECATED: no units specified in ATOMIC_POSITIONS card

     Message from routine read_cards :

     ATOMIC_POSITIONS: units set to alat

     Current dimensions of program PWSCF are:

     Max number of different atomic species (ntypx) = 10

     Max number of k-points (npk) =  40000

     Max angular momentum in pseudopotentials (lmaxx) =  3

               file Sr.pbe-mt_fhi.UPF: wavefunction(s)  5p 4d 4f renormalized

               file Ti.pbe-mt_fhi.UPF: wavefunction(s)  4f renormalized

               file O.pbe-mt_fhi.UPF: wavefunction(s)  4f renormalized

     Message from routine setup:

     no reason to have ecutrho>4*ecutwfc

     G-vector sticks info

     --------------------

     sticks:   dense  smooth     PW     G-vecs:    dense   smooth      PW

     Sum        1741     697    213                54435    13805    2205

     bravais-lattice index     =            1

     lattice parameter (alat)  =       7.3870  a.u.

     unit-cell volume          =     403.0921 (a.u.)^3

     number of atoms/cell      =            5

     number of atomic types    =            3

     number of electrons       =        24.00

     number of Kohn-Sham states=           12

     kinetic-energy cutoff     =      40.0000  Ry

     charge density cutoff     =     400.0000  Ry

     convergence threshold     =      1.0E-08

     mixing beta               =       0.7000

     number of iterations used =            8  plain     mixing

     Exchange-correlation      = PBE ( 1  4  3  4 0 0)

     nstep                     =           50

     celldm(1)=   7.387000  celldm(2)=   0.000000  celldm(3)=   0.000000

     celldm(4)=   0.000000  celldm(5)=   0.000000  celldm(6)=   0.000000

     crystal axes: (cart. coord. in units of alat)

               a(1) = (   1.000000   0.000000   0.000000 ) 

               a(2) = (   0.000000   1.000000   0.000000 ) 

               a(3) = (   0.000000   0.000000   1.000000 ) 

     reciprocal axes: (cart. coord. in units 2 pi/alat)

               b(1) = (  1.000000  0.000000  0.000000 ) 

               b(2) = (  0.000000  1.000000  0.000000 ) 

               b(3) = (  0.000000  0.000000  1.000000 ) 

     PseudoPot. # 1 for Sr read from file:

     C:/cygwin64/home/Markus/QuantumEspresso/espresso-5.4.0/pseudo/Sr.pbe-mt_fhi.UPF

     MD5 check sum: ebef46c480d010156d8a387db00dc5d2

     Pseudo is Norm-conserving + core correction, Zval =  2.0

     Generated using FHI98PP, converted with fhi2upf.x v.5.0.2

     Using radial grid of  537 points,  3 beta functions with:

                l(1) =   1

                l(2) =   2

                l(3) =   3

     PseudoPot. # 2 for Ti read from file:

     C:/cygwin64/home/Markus/QuantumEspresso/espresso-5.4.0/pseudo/Ti.pbe-mt_fhi.UPF

     MD5 check sum: 639edeb71fe77a65403f758b771b521c

     Pseudo is Norm-conserving, Zval =  4.0

     Generated using FHI98PP, converted with fhi2upf.x v.5.0.2

     Using radial grid of  515 points,  3 beta functions with:

                l(1) =   0

                l(2) =   2

                l(3) =   3

     PseudoPot. # 3 for  O read from file:

     C:/cygwin64/home/Markus/QuantumEspresso/espresso-5.4.0/pseudo/O.pbe-mt_fhi.UPF

     MD5 check sum: 2ddb684ddb3c5d7407f47e9cdd68296a

     Pseudo is Norm-conserving, Zval =  6.0

     Generated using FHI98PP, converted with fhi2upf.x v.5.0.2

     Using radial grid of  473 points,  3 beta functions with:

                l(1) =   0

                l(2) =   1

                l(3) =   3

     atomic species   valence    mass     pseudopotential

        Sr             2.00    87.62000     Sr( 1.00)

        Ti             4.00    47.86700     Ti( 1.00)

        O              6.00    15.99940      O( 1.00)

     48 Sym. Ops., with inversion, found

   Cartesian axes

     site n.     atom                  positions (alat units)

         1           Sr  tau(   1) = (   0.5000000   0.5000000   0.5000000  )

         2           Ti  tau(   2) = (   0.0000000   0.0000000   0.0000000  )

         3           O   tau(   3) = (   0.5000000   0.0000000   0.0000000  )

         4           O   tau(   4) = (   0.0000000   0.5000000   0.0000000  )

         5           O   tau(   5) = (   0.0000000   0.0000000   0.5000000  )

     number of k points=     4

                       cart. coord. in units 2pi/alat

        k(    1) = (   0.1250000   0.1250000   0.1250000), wk =   0.2500000

        k(    2) = (   0.1250000   0.1250000   0.3750000), wk =   0.7500000

        k(    3) = (   0.1250000   0.3750000   0.3750000), wk =   0.7500000

        k(    4) = (   0.3750000   0.3750000   0.3750000), wk =   0.2500000

     Dense  grid:    54435 G-vectors     FFT dimensions: (  48,  48,  48)

     Smooth grid:    13805 G-vectors     FFT dimensions: (  30,  30,  30)

     Largest allocated arrays     est. size (Mb)     dimensions

        Kohn-Sham Wavefunctions         0.32 Mb     (    1728,   12)

        NL pseudopotentials             1.61 Mb     (    1728,   61)

        Each V/rho on FFT grid          1.69 Mb     (  110592)

        Each G-vector array             0.42 Mb     (   54435)

        G-vector shells                 0.42 Mb     (   54435)

     Largest temporary arrays     est. size (Mb)     dimensions

        Auxiliary wavefunctions         1.27 Mb     (    1728,   48)

        Each subspace H/S matrix        0.04 Mb     (      48,   48)

        Each matrix      0.01 Mb     (      61,   12)

        Arrays for rho mixing          13.50 Mb     (  110592,    8)

     Initial potential from superposition of free atoms

     starting charge   23.98941, renormalised to   24.00000

     Starting wfc are   80 randomized atomic wfcs

     total cpu time spent up to now is        3.5 secs

     per-process dynamical memory:    28.4 Mb

     Self-consistent Calculation

     iteration #  1     ecut=    40.00 Ry     beta=0.70

     Davidson diagonalization with overlap

     ethr =  1.00E-02,  avg # of iterations =  2.2

     total cpu time spent up to now is        3.8 secs

     total energy              =    -101.59274340 Ry

     Harris-Foulkes estimate   =    -106.54709098 Ry

     estimated scf accuracy   

Well, first are You sure about oxygen positions? The cubic SrTiO3 is FCC I believe, therefore in atomic_species oxygen should look something like this:

O 0.5 0. 0.5

O 0.5 0.5 0

O 0 0.5 0.5

Get rid of those nasty '111' at the end of Atomic_positions card. Also, after 'ATOMIC_POSITIONS' you should specify coordinate system you want to use (crystal, alat etc).

Number two: if you know that your system is rhobohedral near 0K, then you should specify this structure from very beginning. Vc-relax will do it's best only when your starting structure is close enough to the structure you expect - in other words do not expect that vc-relax will find phase transition.

There are generally three situations when forces acting on atoms are exactly 0:

1 - your starting structure is exactly relaxed (very unlikely)

2 - due to symmetry there are no any forces (uncommon)

3 - atoms 'do not feel forces' because they are too far from one another.

Number three: when you check output, you can see that coordinates did changed!

"Begin final coordinates

     new unit-cell volume =    335.14599 a.u.^3 (    49.66351 Ang^3 )

CELL_PARAMETERS (alat=  7.38700000)

   0.940321961   0.000000000   0.000000000

   0.000000000   0.940321961   0.000000000

   0.000000000   0.000000000   0.940321961

ATOMIC_POSITIONS (alat)

Sr       0.470160980   0.470160980   0.470160980

Ti       0.000000000   0.000000000   0.000000000

O        0.470160980   0.000000000   0.000000000

O        0.000000000   0.470160980   0.000000000

O        0.000000000   0.000000000   0.470160980

End final coordinates "

The above is a proof that vc-relax did everything to lower total energy. Look on the resulting 'CELL_PARAMETERS' card (given above, and in your output). QE assumed that your ATOMIC POSITIONS were given in alat coords, following this - the overall cell size (cubic P type) was decreased to about 0.94 of starting one in each direction. The calculated stress was about -343 kbars (witch is quite small). (The symmetric change in atomic positions is probably nbr 2 from list above).

When you tried to visualize your structure, you didn't find changes because:

1. you expected phase transition to rhombohedral - not this case

2. you probably didn't looked for small changes in atomic positions in software used for visualization. 

So, what you should do: check oxygene positions if you want calculate cubic FCC or  change whole structure for expected rhombohedral type. 

Any more questions?

Best regards, good luck and a lot of patience with QE

Konrad

Than you very much Konrad. I will go through your advised steps and answer back....

What is the stress value (-343 kBar) saying actually; Is it the final stress inherent by the final geometry or is it the applied stress from the second last geometry to the final one? Because in equilibrium it should be zero like the forces too. And is -343 kBar really small? I would have thought ist large...

Kind regards

This is indeed final stress (or stress in final optimized cell). But... its value is very low (considering atomic scale)... and you can always tighten force convergence criteria 'forc_conv_thr'.

Okay thanks. I think i'm getting in progress, slowly. The final stress is differing between the final geometry of the vc-relax cycle and the subsequent additional SCF calculation. Is this normal. Depending on the Geometry it differs 300 - 500 kBars....

Yes it is normal. When doing vc-relax there is always some diffrences.  When you do scf after vc-relax, the cell parameters of scf is different from that of vc-relax.

The relax is done for wavefunctions choosen for starting geometry, and because the positions of ions are changing, thus the difference...

Usually, you should do the second vc-relax if the cell parameters change quite a

lot. After the second vc-relax, the force/stress of scf will be close to relax/vc-relax.

Dear Markus Becker

I also recently faced the same problem. But it is solved when I avoid the spin-orbit coupling (SOC) effect from the vc-relax calculation.

Still I'm not satisfied with this way, since I need to consider SOC effect.

Would you share me your experience about how do you solve it in your case?

Regards