i am doing MD simulation, in my case i doing simulation at 300 kel, 500 kel using NPT ensemble. But if i increase temperature in the ensemble, i should also increase the pressure also (i guess) if it is the case how to calculate the pressure. Can i calculate the mechanical properties at higher temperatures of al2O3 . Can i derive the mechanical properties . Plz share the information.
thank you
Dear Harsha Praneeth,
as you know in a NPT System no. elements, pressure and temperature are controlled e.g. they are kept constant. Hence whatever pressure and temp you choose the program will adjust size (volume V, shape) and internal energy (U) to follow this concept.
So for a given temp you do not Need to Change your pressure as the program will adjust to the defined pressure by volume Expansion. The volume adjustment at constant pressure (!) is equivalent to enthalpy changes (volume work) as the constant temp requires entropy adjustments (heat flow in / out of the System by the changing Arrangement / structure).
The Major Problem will be your Al2O3 potential function (if it is stable, in particular for the Dynamics of the MD calculation at 300K and 500K).
what Kind of properties you want to calculate?
Regards, Ruediger
i want mechanical properties of the material. My question is if increase the temperature of 300 kel with a presuure of 0.1 to 400 kel can i keep the same pressure values and do the simulation or is there a way to calculate the pressure.
dear Harsha Praneeth, if you are using a NPT code, the program will Keep the specified pressure (which you choose as Input value, just as the temperature as well). The NPT code Needs to calculate the pressure in the box or volume and adjusts the volume so as to Keep the desired pressure.
Hence a Change from 300K to 400K at pressure 0.1 you should be able to calculate. Btw make sure that you have the right pressure (check the Units); I remember that some codes require the pressure Input in GPa (!) However there will be most likely no significant Change for such a rather small temp difference. it will expand to some degree (check the absolute total volume of the box), but it will even Keep the same structure I guess). Since you deal with a solid material with rather high bonding energies, only significant temperature ranges (Close to melting temperature) will bring visible changes to the structure. You could, of course observe the pressure in your volume, simply by plotting out the calculated pressure values which will be compared to the desired pressure. Calculating typical macroscopic mechanical properties (like Engineering properties) is not necessarily an easy Task, as all to often, the results are not the same (because of the finite size of your most idealized structure. What properties do intend to calculate?
Best regards, R. Rentsch
i want to find the variation in youngs modulus, bulk modulus, shear modulus (E,G,K, mu). I going up to 1200 kelvin. I have observed a drop in mechanical properties by 30% sir. I have taken some cif files and calculated the mechanical properties at elevated temperatures. Many researchers came up with the mechanical properties at 300 kel and it is published any i using the same methodology and scaling up the temperature to 1200 kel. Plz give me any input sir, as the unit cells bonding gets effected there is some variation in stiffness matrix which gives us the mechanical properties. If i am wrong plz share your opinion sir and its very valuable for me . Thank you for your response sir.
dear Harsha Praneeth, theresults will depend very much on the boundary conditons that you apply and the structure that you are using. An ideal structure e.g., with no surfaces and periodic boundary conditions (PBC) will lead to the highes values which will beyond all that you can measure in experiments. When you write "you observed" are you talking about experiments or simulations? Are you going to compare your Simulation results to experimental results from the literature? What do you mean by "unit cells bonding"?
Regards, R. Rentsch
By the way, if you like the answers then vote them up!
I have taken a unit cell from American crystallography file and transferred the CIF file to material studio and it applies periodic boundary condition, and I am doing simulation only sir. Because at nanoscale physical experiments are not possible ( mostly I guess ).
Dear Harsha,
Just saw your query. You can find NPT and NVT input files of IFF in our recently developed database.
https://cemff.epfl.ch/home/.
FYI, I have added an input file for Young's modulus calculation in x-direction. But you need to run at different uniaxial stress to plot stress vs. strain plot. Then you can calculate YM by computing the slope of the graph.
For bulk mod. calculation, you need to vary triaxial stresses. Please go through our FF validations papers.
http://pubs.rsc.org/-/content/articlehtml/2014/dt/c4dt00438h
Let me know if you have any further problem.
thanks rathan k mishra for your file please share your jounral you have attached, it would be helpful for me i guess.
thanks
All articles are available in my researchgate profile. Please go through it.
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