[Pw_forum] Initial geometry optimization cannot converge

[Stefano de Gironcoli]

Dear Chenyang Lee,
 -  You are using a damp vcrelax scheme that requires some little experience to fix the time step and the cell mass. It might not be the reason for the slow convergence but I would give a try to the bfgs algorithm that does not need those input variables.
- Also, I would start relaxing the internal coordinates at first and switch on the cell motion when atoms are in somewhat reasonable positions. This is not strictly necessary but if your relaxation shows wild volume oscillations I would consider that.
- As you say your structure can be far from the equilibrium one and your cell is not small I would use many less k-point, even just Gamma, for the first round of optimization and switch to more dense grid for the final refinement.

stefano 

> On 25 Feb 2015, at 03:18, Chenyang Lee <lee19921027 at gmail.com> wrote:
> 
> Dear experts in PWforum,
> 
> I am experiencing difficulties relaxing some of the structures of B-HAp. Because the Boron group substitutions are proposed (unknown), thus the initial structure may deviate a lot from the final optimized one. Quantum Espresso seems not to be very capable to do these jobs (no offense) and the SCF cycles usually exceed 500 steps and it still cannot converge. 
> I am attaching the input file, and I’d appreciate it if someone who is able to identify the problem can help me solve it.
> 
> Thank you very much,
> Chenyang
> __
> Chenyang Li
> Undergraduate Student, Materials Science and Engineering
> The Pennsylvania State University, University Park
> Phone: (814)441-4312
> 
> _______________________________________________
> 
> &control
>    calculation = 'vc-relax'
>    restart_mode = 'from_scratch'
>    prefix = ‘BHAp'
>    pseudo_dir = '/gpfs/home/col5422/work/HAp/pseudo/'
>    outdir = './scratch/'
>    tprnfor = .true.
>    forc_conv_thr = 1e-4
>    etot_conv_thr = 1e-6
>    nstep = 500
> /
> &system
>    ibrav = 14
>    A = 9.4166
>    B = 9.4166
>    C = 6.8745
>    cosAC = 0.0
>    cosAB = -0.5
>    cosBC = 0.0
>    nat = 52
>    ntyp = 6
>    ecutwfc = 40
>    ecutrho = 400
>    nosym = .true.
>    occupations = 'smearing'
>    smearing = 'gaussian'
>    degauss = 0.02
> /
> &electrons
>    electron_maxstep = 500
>    diagonalization = 'david'
>    mixing_mode = 'plain'
>    mixing_beta = 0.3
>    conv_thr = 1e-6
> /
> &ions
>    ion_dynamics = ‘damp'
> /
> &cell
>    cell_dynamics = ‘damp-pr'
> /
> 
> ATOMIC_SPECIES
> H   1  H.pbe-kjpaw_psl.0.1.upf
> B   1  B.pbe-n-kjpaw_psl.0.1.upf
> C   1  C.pbe-n-kjpaw_psl.0.1.upf
> O   1  O.pbe-n-kjpaw_psl.0.1.upf
> P    1  P.pbe-n-kjpaw_psl.0.1.upf
> Ca  1  Ca.pbe-spn-kjpaw_psl.0.2.3.upf
> 
> ATOMIC_POSITIONS crystal
> H     0.000000   0.000000   0.400000
> H     0.000000   0.000000   0.900000
> H     0.567369   0.551274   0.972591
> H     0.629383   0.851436   0.797858
> H     0.357078   0.434312   0.660517
> H     0.809237   0.663097   0.589841
> B     0.581602   0.618142   0.689068
> C     0.310984   0.129561   0.635342
> O     0.348000   0.259000   0.073000
> O     0.741000   0.089000   0.073000
> O     0.911000   0.652000   0.073000
> O     0.259000   0.911000   0.573000
> O     0.089000   0.348000   0.573000
> O     0.259000   0.911000   0.927000
> O     0.089000   0.348000   0.927000
> O     0.348000   0.259000   0.427000
> O     0.741000   0.089000   0.427000
> O     0.911000   0.652000   0.427000
> O     0.329000   0.484000   0.250000
> O     0.589000   0.466000   0.250000
> O     0.516000  -0.155000   0.250000
> O     0.534000   0.123000   0.250000
> O     1.155000   0.671000   0.250000
> O     0.877000   0.411000   0.250000
> O     0.484000   1.155000   0.750000
> O     0.466000   0.877000   0.750000
> O    -0.155000   0.329000   0.750000
> O     0.123000   0.589000   0.750000
> O     0.000000   0.000000   0.250000
> O     0.000000   0.000000   0.750000
> O     0.603664   0.524999   0.843110
> O     0.718661   0.681136   0.557507
> O     0.546871   0.739153   0.774413
> O     0.441199   0.509454   0.566674
> O     0.464549   0.231819   0.634250
> O     0.235429   0.067896   0.472588
> O     0.237756   0.091652   0.804032
> P     0.400000   0.369000   0.250000
> P     0.631000   0.031000   0.250000
> P     0.969000   0.600000   0.250000
> P     0.369000   0.969000   0.750000
> P     0.031000   0.400000   0.750000
> Ca    0.333333   0.666667   0.001000
> Ca    0.666667   0.333333   0.501000
> Ca    0.666667   0.333333   0.999000
> Ca    0.333333   0.666667   0.499000
> Ca    0.246000   0.993000   0.250000
> Ca    0.007000   0.253000   0.250000
> Ca    0.747000   0.754000   0.250000
> Ca    0.754000   0.007000   0.750000
> Ca    0.993000   0.747000   0.750000
> Ca    0.253000   0.246000   0.750000
> 
> K_POINTS automatic
> 4 4 4 1 1 1
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