ERROR : Cell parameter has fallen below allowed limit and Elastic Constant Matrix error value

GULP
1

Hello all., I am trying to find elastic constants for Sr2MgSi2O7 sample. From the cif file i have substituted lattice parameters and fractional coordinates, Species, and buckingham potential were taken from ucl website. I am facing flollowing issues -

  1. I was getting charge is not neutral error, then i saw in manual and included ‘qok’ command, then error disappered.

  2. Yet, I am getting Cell parameter has fallen below allowed limit and elastic constants matrix showing **** values.

  3. Herewith I am attaching .cif file (Kindly help which is core and shel in fractional coordiantes)
    Thank you all…

  4. GULP INPUT
    opti prop conp qok
    title
    Sr2 test file
    end
    cell
    8.09718 8.09718 5.22654 90 90 90
    frac
    Sr0 core 0.66614400 0.16614400 0.50787900
    Sr1 core 0.83385600 0.66614400 0.49212100
    Sr2 core 0.16614400 0.33385600 0.49212100
    Sr3 core 0.33385600 0.83385600 0.50787900
    Mg4 core 0.00000000 0.00000000 0.00000000
    Mg5 core 0.50000000 0.50000000 0.00000000
    Si6 core 0.86130200 0.36130200 0.94352300
    Si7 core 0.13869800 0.63869800 0.94352300
    Si8 core 0.63869800 0.86130200 0.05647700
    Si9 core 0.36130200 0.13869800 0.05647700
    O10 core 0.68990400 0.42045000 0.80149600
    O11 core 0.31009600 0.57955000 0.80149600
    O12 core 0.42045000 0.31009600 0.19850400
    O13 core 0.57955000 0.68990400 0.19850400
    O14 core 0.92045000 0.18990400 0.80149600
    O15 core 0.81009600 0.92045000 0.19850400
    O16 core 0.18990400 0.07955000 0.19850400
    O17 core 0.07955000 0.81009600 0.80149600
    O18 core 0.85997100 0.35997100 0.25353600
    O19 core 0.64002900 0.85997100 0.74646400
    O20 core 0.50000000 0.00000000 0.16477200
    O21 core 0.00000000 0.50000000 0.83522800
    O22 core 0.14002900 0.64002900 0.25353600
    O23 core 0.35997100 0.14002900 0.74646400
    space
    P 1
    Species
    Sr core 2.00000
    Mg core 2.00000
    Si core 4.00000
    O core 0.86902
    buckingham
    Sr shel O shel 1952.390 0.33685 19.22000 0.0 10.0
    Mg shel O shel 946.627 0.31813 0.00000 0.0 10.0
    Si shel O shel 1283.907 0.32052 10.66158 0.0 10.0
    O shel O shel 22764.000 0.14900 27.87900 0.0 10.0
    spring
    Sr 21.53
    Mg 349.95
    Si 10.00
    O 74.92
    output xr example1
    output marvin example1.mvn

GULP ERROR
ERROR : Cell parameter has fallen below allowed limit

Also
Piezoelectric Strain Matrix: (Units=C/m**2)

Indices 1 2 3 4 5 6

   x  ************************************************************
   y  ************************************************************
   z  ************************************************************

Elastic Constant Matrix: (Units=GPa)

Indices 1 2 3 4 5 6

   1  ************************************************************
   2  ************************************************************
   3  ************************************************************
   4  ************************************************************
   5  ************************************************************
   6  ************************************************************

Elastic Compliance Matrix: (Units=1/GPa)

Indices 1 2 3 4 5 6

   1   -0.000000 -0.000000 -0.000000 -0.000000  0.000000 -0.000000
   2   -0.000000 -0.000000 -0.000000  0.000000 -0.000000 -0.000000
   3   -0.000000 -0.000000 -0.000000  0.000000  0.000000  0.000000
   4   -0.000000  0.000000  0.000000 -0.000000 -0.000001  0.000000
   5    0.000000 -0.000000  0.000000 -0.000001 -0.000000  0.000000
   6   -0.000000 -0.000000  0.000000  0.000000  0.000000  0.000000
2

The first and most important thing to say here is, never perform a constant pressure optimisation on a non-charge neutral system unless you really know what you are doing and fully understand the consequences and theory behind this because bad things can happen (as you’ve seen). If you think about it, a periodic infinitely charged system should explode & so crazy results should come as no surprise. The qok keyword is to allow defect calculations to be performed in a supercell and not as a fix for getting the input wrong such that the charges don’t add up. You need to work out what is wrong with your input and fix it (i.e. why is it not charge neutral?). If you compare to other examples that come with GULP of shell model calculations (e.g. example1) you should be wondering why you have no shells in your input…

3

Thank you very much for your valuable information. Kindly let me know to make charge neutral, it is not right to vary fractional coordinates, that may change the position of atoms for a particular comound, I hope we should always retain the data of cif file. The only possiblity to refine the input file is by varying buckingham potential or species, but changing potential is not varying the sum of charges, Kindly suggest me…