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1:30 PM
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A: Why are DFT simulations/calculations called First Principles Structural calculations? What is 'First Principles'?

SaiSmaran S B PES1201701189PESFirst Principles mean starting directly at the level of established science and not making assumptions such as any empirical models or parameter fitting. With respect to DFT, EMF (Electromagnetic force) is a very strong force governing nucleus and electrons (referring to a single atom). With tha...

 
Ruslan
Ruslan
 
SaiSmaran S B PES1201701189PES
SaiSmaran S B PES1201701189PES
The calculation methods are based on quantum mechanics, therefore, do not use any fitting parameters from the experimental data as used in MD(Molecular Dynamics) and are based solely on the basic laws of physics. You can refer books.google.co.in/books/about/…
 
Ruslan
Ruslan
You still do need input data that would influence whether you're modeling e.g. 3O₂ or 2O₃ system. And that can actually rely on experimental parameters like bond lengths or whatever.
 
SaiSmaran S B PES1201701189PES
SaiSmaran S B PES1201701189PES
I think those parameters directly depend on the atomic number.
 
Ruslan
Ruslan
In the example system there is only one atomic number: 8, and there are 6 of these atoms. Their arrangement defines whether it's ozone or oxygen.
 
SaiSmaran S B PES1201701189PES
SaiSmaran S B PES1201701189PES
1:30 PM
Oh okay, So what edit to the answer would you suggest?
 
Ruslan
Ruslan
I don't know, I wanted you to tell what exactly parameters are acceptable in ab initio calculations and what aren't. But rejecting all parameters is obviously wrong.
 
 
7 hours later…
Camps
Camps
8:14 PM
In general, the only information used in DFT are the type of atoms.

It is not a true first principle because it uses approximations like LDA, GGA and functionals (vdW, PBE, etc.) that can be developed using some experimental data.

In order to do a calculation, a starting structure must be used as input. That's the subject of (When do I use the lattice parameters obtained computationally vs experimentally?)[https://mattermodeling.stackexchange.com/q/2405/721]
 

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