High pressure Calculation

[Inaoton]

hello Sir,
                   This compound when applied pressure undergoes phase transition at high pressure.  So i have constructed the two phases and
           and tried to  calculate the enthalpies of the  phases from 0 to 70Gpa(Using OptimizeGeometry). When i am plotting  a graph of
           enthalpy vs Pressure(attached), the Nacl structure is showing me the correct response(as I think, It is following the trend of a reference ) for the complete range of pressure applied whereas the bct  structure is  giving me an error from 55Gpa Onwards.
           I have cross checked the result at 55Gpa for bct again and again but the enthalpy never changes .
           1. Is it related to some software issue ?
           2. Can you just have a look at my script? (attached)
                How I rectify the problem? (I am confused)

[Anders Blom]

What "error"? Is there an error message in the output file? What is the result at 60?

[Inaoton]

No there is no error message in the output file
The enthalpy at 60Gpa is -19081.705425 eV and it keeps on increasing but the enthalpy value of bct should be more than that of Nacl structure(in all cases after 45Gpa)
Is there something i need to change in Optimizer input for calculation of high pressure?

[Anders Blom]

The enthalpy value of bct should be more than that of Nacl structure(in all cases after 45Gpa)

Why?

Is there something i need to change in Optimizer input for calculation of high pressure?

Not from a technical perspective. You have good parameters generally, low convergence tolerance and you use the high SG15 basis sets, so all that looks good. But of course, at high-pressure, maybe the DFT model is not as accurate, maybe the basis set is not so good, maybe LDA instead of GGA would be better,  there are many things to explore. I would also try to not run 55 GPa from the zero-pressure structure, but use that calculated at 50 GPa as starting point, it would probably be faster, and maybe you don't get stuck in a (different) local minimum by bad luck, but rather continue along the trajectory of lower pressure, into the same symmetry.

[Inaoton]

Sir
       Since the first phase(Bct) was becoming unstable due to increasing pressure applied(enthalpy increases),  it undergoes phase transition (the line intersects around 40Gpa). So my hypothesis is that the  second phase ( i.e. The Nacl phase ) will be  more stable after the transition pressure and hence lower enthalpy after 40Gpa as it is showing for 45Gpa and 50Gpa. But something is happening wrong at 55Gpa.
       I have calculated the same at 150 and 200 Hartree also. I am getting the same answer.
      1. Is there a possibility that using a higher max_step _length(default 0.2Angstrom) in optimizer has avoided in attaining the minimum?
      2.Is it possible  that i first optimize stress of a structure first and then optimize force later on ?
     
     
       

[Jess Wellendorff]

1: I don't think so. The calculation should not be sensitive to that parameter.
2: If you set the Force convergence criterion to something very high, e.g. 1000 eV/Ang, you should get a GeometryOptimization where only the stress is really relaxed.

[Anders Blom]

You mentioned you already tried a different density mesh cut-off, that is a good idea, but maybe an even higher value is needed, 200 is not extremely high and when applying pressure, you may need a very detailed description.

Maybe I was not so clear about the point of rerunning from the 50 GPa geometry. It is possible (perhaps not very likely, but possible) that starting from the zero-pressure structure, your 50 GPa and 55 GPa calculations end up in different (local) minima. You could try to evaluate the space group symmetry of each result and compare, and as mentioned instead of starting always from the same, use the 50 GPa converged geometry when applying 55 GPa pressure.