How to obtain the spectrum for a given atom or simple molecule?

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The Bohr atom gave the answer to the spectrum of the hydrogen atom.

But the spectra of stars contains many absorption (and sometimes emission) lines, corresponding to most atoms (up to iron, I believe).

And atmospheric absorption is also due to absorption of some molecules, such as water, carbon dioxide and, I assume, some nitrogen molecule.

My question is: where can I find information to find methods (which I assume are numerical, say Hartree-Fock or extensions thereof) that are able to determine/predict/posdict the absorptions of a given atom and molecule, including electronic / vibration / rotation / ... modes?
 

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  • #2
DrClaude
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What is needed is computational chemistry software, see
https://en.wikipedia.org/wiki/List_of_quantum_chemistry_and_solid-state_physics_software
Note that getting good numbers can be quite involved, especially for more complicated systems.

It is often best to rely on experimental spectroscopic data.
Thank you. Is there a book on both the underlying methods, and the practical usage of this software? I have heard about CP2K and Gaussian.
 
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DrClaude
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Is there a book on both the underlying methods, and the practical usage of this software?
I don't know if there is a book that covers both. This software is mostly targeted at researchers, so a certain base knowledge is assumed. Also, many different approaches can be used, from semi-empirical methods to density functional theory, so I don't think you can get it all in one source.

For a very basic introduction to the subject, there is a chapter on computational chemistry in Atkins and Friedman's Molecular Quantum Mechanics. Likewise, a basic introduction can be found in Schrier, Introduction to Computational Physical Chemistry. A more complete description of Hartree-Fock methods can be found in McWeeny, Methods of Molecular Quantum Mechanics (it is graduate level).

Thinking about it, there would be one book that covers method and software, which is MOTECC. You might be able to find it in a university library, but I don't know if it is possible to get the software that is described in there.
https://books.google.com/books?id=vZlR9sXdGowC&pg=PA12&lpg=PA12&dq=motecc&source=bl&ots=dqaOaiADpS&sig=ACfU3U059qd5H3ge6vwxqCGZASChEiG5bw&hl=en&sa=X&ved=2ahUKEwiM4aGljZHiAhWxlYsKHV_-DfwQ6AEwA3oECAkQAQ#v=onepage&q&f=false
 
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ZapperZ
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The Bohr atom gave the answer to the spectrum of the hydrogen atom.

But the spectra of stars contains many absorption (and sometimes emission) lines, corresponding to most atoms (up to iron, I believe).

And atmospheric absorption is also due to absorption of some molecules, such as water, carbon dioxide and, I assume, some nitrogen molecule.

My question is: where can I find information to find methods (which I assume are numerical, say Hartree-Fock or extensions thereof) that are able to determine/predict/posdict the absorptions of a given atom and molecule, including electronic / vibration / rotation / ... modes?
As has been mentioned, there is no one book that covers everything. In fact, there is no one method that can numerically calculate every single transition for every single atom and molecules. It is why things like DFT often come up in many of such computation, etc.

BTW, most of the spectra (emission and absorption) that are currently used as the "fingerprint" of various atoms are derived from empirical data, not solely from computational results.

Zz.
 
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Thank you for the answer. Which are the limits of spectral calculations? Are there some materials that are not tractable even by numerical calculations?
 
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DrDu
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Thank you for the answer. Which are the limits of spectral calculations? Are there some materials that are not tractable even by numerical calculations?
I'd say hyperfine structures would be very difficult.
 
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Thank you for the answer. Which are the limits of spectral calculations? Are there some materials that are not tractable even by numerical calculations?
But the spectra of stars contains many absorption (and sometimes emission) lines, corresponding to most atoms (up to iron, I believe).
 
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But the spectra of stars contains many pornjk porn800 redtube absorption (and sometimes emission) lines, corresponding to most atoms (up to iron, I believe).
Thinking about it, there would be one book that covers method and software, which is MOTECC. You might be able to find it in a university library, but I don't know if it is possible to get the software that is described in there.
 
  • #11
Dr. Courtney
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Thank you for the answer. Which are the limits of spectral calculations? Are there some materials that are not tractable even by numerical calculations?
Quantum mechanics works and makes accurate predictions. The challenge with more complex atoms and molecules is that the application of perturbation theory applying quantum mechanics to these complex systems requires including an an infinite number of quantum states and diagonalizing infinite matrices. Computers cannot do this. In practice, those doing the calculating truncate the number of quantum states to a finite number.

Using better computational techniques can squeeze more accurate predictions from a given number of quantum states included, but at any given "state of the art" of computing power and clever computational techniques, there is a threshold of system complexity whose energy levels (thus spectra) can be computed accurately. Accurate computations above that level of complexity will require more computing power and the development of more clever techniques.

For many atoms and molecules, it is still simply easier to measure the spectra with modern spectroscopic techniques.
 

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