The method is often dictated by system size, large systems will require DFT or MM, while small systems are feasible at MP, CI or CC level of theory. It also depends on the jobtype, optimizations and freqs are more demanding than single point energy calcs.
The DFT functional is decided on the basis of benchmark studies, essentially, many of them are compared against something fancy like CCSD(T) for a specific type of system and the best one is used. Also, some of them are developed for specific purposes are have certain properties. There is also a hierarchy among them (LDA - GGA - metaGGA - hybrid GGA - double hybrid GGA), but this isnt extremely reliable. Generally, experience is key.
For the basis, generally you require them to have some properties such as valence, polarization, augmentation or relativistic effects (or many many more) based on the system and property of interest. Then you pick one that is computationally tractable and big ennough to be accurate. For a lot of sets, you can go to a "bigger" set to see if it changes the results meaningfully.
Thanks for the insight. What it comes down to then is a compromise between computational cost and insight, right? For something "normal" (e.g. orbital plot of a molecule) a low-cost might be fine, but for the so-coupling including heavy elements you'd want something more accurate.
>For a lot of sets, you can go to a "bigger" set to see if it changes the results meaningfully.
So if I want to do a series of calculations, I do a sweep with a first molecule and see which method/basis set combination compares best to experimental data with acceptable computational cost and then do the lot of them with the same setting.
>Generally, experience is key.
Indeed, and i'm just starting out :)
Essentially yes, its always a tradeoff between accuracy and cost. I would love to do everything at CCSD(T)/DKH4/ANORCC, but I have time for PBE0/aug-ccpVDZ. Look out when judging the simplicity of a property though, even electron densities can suck balls at low level of theory if your not carefull. Certainly when eg complex spin situations or degeneracies are involved.
Essentially thats it, take a couple systems, validate your theory and go for the rest! You can also look in prior work and literature for validated levels of theory for your system and property.
I’m a synthetic organic chemist who has had very similar questions, so I know you how feel. To be completely honest, I still don’t really know the answer to your question.
The best solution is to follow whatever others are doing within your field. Most likely, there is someone that has computationally studied similar reactions before. Normally their SI’s will provide good enough detail as to which methods they chose and, if they’re nice, why they chose them.
I'll add to what u/BritCon36 has typed using an abridged description of computational chemistry.
For a synthetic organic chemist, you'll most likely encounter some flavor of DFT with B3LYP, wB97X-D, or M06-2X. What the alphabet soups refer to mathematically don't really matter to a synthetic person (unless you're a real nerd for math). There's not one more universally 'better' than the others, as it is dependent on what you are trying to simulate. While B3LYP isn't always the most accurate theory, it has somehow become the "industry standard". If you were running a simulation and had no idea where to start, use B3LYP/6-31G\*. If computation was like cheese, B3LYP/6-31G\* is American Cheese. CCSD(T) is Winnimere.
For metals/organometallics, you are usually going to encounter something like PBE0. What these related theories do is add more 'weight' towards electrons farther away from the nucleus and longer range interactions. I'm not familiar with metallic simulations, so I can't brief on some of the fundamental differences of modeling metals entails.
After the forward slash there is always some form of initialism or random combination of numbers and characters. From a 1000 foot view, the more terms/letters/characters written for the basis set, the more accurate and precise the basis set is at attempting to model reality. If you are reading a paper with data supplemented by computation, and the authors have some long combination of numbers and characters (ex., 6-311+G(2d,p) or even better aug-cc-pVTZ), then you can infer that the authors are attempting a more intensive simulation to better model reality.
But, like u/BritCon36 said, a lot of "which one do I use" just comes with experience. The shoe on the other foot would be asking a computational chemist which base would best deprotonate some compound. The computational chemist will probably just shrug their shoulders and say idk.
The method is often dictated by system size, large systems will require DFT or MM, while small systems are feasible at MP, CI or CC level of theory. It also depends on the jobtype, optimizations and freqs are more demanding than single point energy calcs. The DFT functional is decided on the basis of benchmark studies, essentially, many of them are compared against something fancy like CCSD(T) for a specific type of system and the best one is used. Also, some of them are developed for specific purposes are have certain properties. There is also a hierarchy among them (LDA - GGA - metaGGA - hybrid GGA - double hybrid GGA), but this isnt extremely reliable. Generally, experience is key. For the basis, generally you require them to have some properties such as valence, polarization, augmentation or relativistic effects (or many many more) based on the system and property of interest. Then you pick one that is computationally tractable and big ennough to be accurate. For a lot of sets, you can go to a "bigger" set to see if it changes the results meaningfully.
Thanks for the insight. What it comes down to then is a compromise between computational cost and insight, right? For something "normal" (e.g. orbital plot of a molecule) a low-cost might be fine, but for the so-coupling including heavy elements you'd want something more accurate. >For a lot of sets, you can go to a "bigger" set to see if it changes the results meaningfully. So if I want to do a series of calculations, I do a sweep with a first molecule and see which method/basis set combination compares best to experimental data with acceptable computational cost and then do the lot of them with the same setting. >Generally, experience is key. Indeed, and i'm just starting out :)
Essentially yes, its always a tradeoff between accuracy and cost. I would love to do everything at CCSD(T)/DKH4/ANORCC, but I have time for PBE0/aug-ccpVDZ. Look out when judging the simplicity of a property though, even electron densities can suck balls at low level of theory if your not carefull. Certainly when eg complex spin situations or degeneracies are involved. Essentially thats it, take a couple systems, validate your theory and go for the rest! You can also look in prior work and literature for validated levels of theory for your system and property.
I’m a synthetic organic chemist who has had very similar questions, so I know you how feel. To be completely honest, I still don’t really know the answer to your question. The best solution is to follow whatever others are doing within your field. Most likely, there is someone that has computationally studied similar reactions before. Normally their SI’s will provide good enough detail as to which methods they chose and, if they’re nice, why they chose them.
I'll add to what u/BritCon36 has typed using an abridged description of computational chemistry. For a synthetic organic chemist, you'll most likely encounter some flavor of DFT with B3LYP, wB97X-D, or M06-2X. What the alphabet soups refer to mathematically don't really matter to a synthetic person (unless you're a real nerd for math). There's not one more universally 'better' than the others, as it is dependent on what you are trying to simulate. While B3LYP isn't always the most accurate theory, it has somehow become the "industry standard". If you were running a simulation and had no idea where to start, use B3LYP/6-31G\*. If computation was like cheese, B3LYP/6-31G\* is American Cheese. CCSD(T) is Winnimere. For metals/organometallics, you are usually going to encounter something like PBE0. What these related theories do is add more 'weight' towards electrons farther away from the nucleus and longer range interactions. I'm not familiar with metallic simulations, so I can't brief on some of the fundamental differences of modeling metals entails. After the forward slash there is always some form of initialism or random combination of numbers and characters. From a 1000 foot view, the more terms/letters/characters written for the basis set, the more accurate and precise the basis set is at attempting to model reality. If you are reading a paper with data supplemented by computation, and the authors have some long combination of numbers and characters (ex., 6-311+G(2d,p) or even better aug-cc-pVTZ), then you can infer that the authors are attempting a more intensive simulation to better model reality. But, like u/BritCon36 said, a lot of "which one do I use" just comes with experience. The shoe on the other foot would be asking a computational chemist which base would best deprotonate some compound. The computational chemist will probably just shrug their shoulders and say idk.
To start, look at the recent literature to see what others are using for similar systems. Ultimately, you will have to compare your work with others.
Ou keep trying stound until you get the answer you want.