[Wien] Murnaghan equation

[Stefaan Cottenier]

>   I am optimizing the volume of a hexagonal cell.  I changed the   
> volume -9 to 0% reduction with step of -1 in optimize.job.  I got   
> the smooth parabola.  The minimum energy lies close to the volume of  
>  -5% reduction.  Volume at -5% reduction is V=999.6100. In Murnaghan  
>  equation, minimum energy volume Vo=990.8361.  I read your reply   
> about murnaghan question in   
> http://zeus.theochem.tuwien.ac.at/pipermail/wien/2006-November/008287.html.
>   So Murnaghan equation gives minimum energy volume correctly.
>   But after that i want to optimize c/a ratio.  I had chosen the the  
>  case_vol__-5.0.struct file to undergo change in c/a ratio at   
> constant volume.  Because i don't have the struct file for volume Vo  
>  showed by Murnaghan equation.
>   am i in correct direction?

Nothing prevents you from making a case.struct with the optimal volume  
and same c/a as your initial series, and start the c/a optimization  
from there on. Once found the optimal c/a at that volume, make again a  
cell with that optimal c/a, and optimize the volume again, etc. etc.,  
until nothing changes any more (often 2 such iterations are sufficient).

Alternative: optimize the volume for e.g. 5 different c/a ratio's, and  
determine the absolute minimum in an Energy versus (volume, c/a) plot  
(3D plot).

Stefaan



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