# Bulk Modulus from DFT results

1. Oct 13, 2008

### raovq

1. The problem statement, all variables and given/known data
I have been asked to calculate the bulk modulus for a carbon dioxide solid from data I have calculated. I have made a quick plot of unit cell size vs pressure (sorry for no labels, unit cell is Y axis as square angstroms, and pressure is in GPa), which looks ok.

http://img406.imageshack.us/img406/1072/screenshotin1.png [Broken]

2. Relevant equations
I have been told to fit it to the http://en.wikipedia.org/wiki/Birch–Murnaghan_equation_of_state" [Broken] but that is where it all falls over (I'm a chemist completely out of my depth).

I have been told I can also "You can finite difference the pressure vs volume at low pressure to get dP/dV and then the BM can be got from BM = - V(dP/dV)", but I am not sure what that actually means.

I'm not fussed which method I use, the room for error on this particular example is acceptable at around 50 GPa (It will tell me if I am even remotely close to having what i want, which is 350 GPa).

3. The attempt at a solution
It seems that most people do it with scripts, but most I have seen rely on preforming thier own calculations to get the raw data. As a condition of this project I have to use a specific application, which seems to be the easy part.

Thanks for any advice, I can't seem to make any sense from any papers I have read so far, and it is starting to look a little sad.

Last edited by a moderator: May 3, 2017
2. Oct 13, 2008

### Redbelly98

Staff Emeritus
An important question is, what is the unit cell's volume given your square-Angstrom values?

If you can get that, then a curve fit will give you dV/dP at any pressure.

Once you have dV/dP, then use your equation to get BM.