From owner-chemistry ^%at%^ ccl.net Thu Sep 10 07:42:00 2015 From: "Peter Jarowski peterjarowski|*|gmail.com" To: CCL Subject: CCL: Case Studies of QM Computational Chemistry in Reactivity Message-Id: <-51690-150910031216-10633-4s151BQXxUrvrjgJG9iAzw]^[server.ccl.net> X-Original-From: Peter Jarowski Content-Type: multipart/alternative; boundary=001a1141f37e502c9a051f5f51e9 Date: Thu, 10 Sep 2015 09:12:10 +0200 MIME-Version: 1.0 Sent to CCL by: Peter Jarowski [peterjarowski+*+gmail.com] --001a1141f37e502c9a051f5f51e9 Content-Type: text/plain; charset=UTF-8 Hi Lars: There is no need for defense. We all agree that method development is important. Personally, I am a bit defensive of my own work which was published in 2009 in a non-theoretical journal and consist of 80% experiment. I provided that example to show one where theory (whatever it is) worked well to the point of prediction. Again, this is what I am looking for in this thread. Unfortunately, theory can not exist on its own and the finances flow from experiment downward. You will find only blank looks with the arguments you have made outside of a room full of theorists and that is the reality. On the other hand, often we are in front of chemists, at least, and having verified chemical examples of the importance of theory in necessary. These should be chemically understandable and should be dealing with issues that are important to the audience. They should be transferable. That means using standard methods with the right statistics and history that can be computed in a reasonable time and executed, potentially, by non-experts. So, while I love MRCI, for example, even the best of us will have trouble getting it right on relevant molecules. Best, Peter On Wed, Sep 9, 2015 at 4:06 PM, Lars Goerigk lars.goerigk-,-unimelb.edu.au < owner-chemistry**ccl.net> wrote: > > Sent to CCL by: "Lars Goerigk" [lars.goerigk : unimelb.edu.au] > Hi Peter, > > in defense of us people that believe in obtaining the right result for the > right reason: even if industry is interested in quick-and-dirty results, > would it not be highly embarassing to make recommendations based on a > method that relies on unforeseeable error compensation, only to then see > that the experimentalists cannot reproduce your predictions? Susi's comment > was therefore valid, albeit I would have worded it in a more diplomatic way. > > I find it very important to apply a level of theory that comes with a low > risk of unsystematic error compensation. For example, a > dispersion-corrected (double-)hybrid density functional with at least a > triple-zeta basis set is overall much more reliable than the popular > B3LYP/6-31G* level; the reasons for that are clear and they have been > discussed extensively in the literature and in this forum. If you cannot > afford a large basis set, but you want to quickly obtain results for bigger > systems with an acceptable accuracy, there are other promising methods out > there (e.g. HF-3c or PBEh-3c). Moreover, it is also important to use > reliable solvation models (e.g. COSMO-RS) and reliable enthalpy and entropy > corrections. > > As to your comment on "level nerds" being a barrier for successful > interaction between theory and experiment, quantum chemists do not > investigate and develop new methdods just because they do not know what > else to do with their time. I would rather say that this discussion > highlights a lack of communication between method developers and the users > of QM methods and as always the fault for that lies probably on both sides; > one consequence of that is that B3LYP/6-31G* is still so popular in the > year 2015. > > Giving a general answer to your question is difficult, as it is not quite > sure what property you want to calculate, which is exactly why the various > answers to your question differ so much. However, there are numerous > benchmark studies out there that provide some guidance on which methods to > use and which to avoid; I have given some hints on reliable methods above. > > Of course, there is never 100% certainty that your predictions are right > because in the end you try to describe a highly complex system with a > theoretical model, which by definition is only a simplification of the real > world. That complexity also means that there is not one sole, easy solution > to your question. Therefore, it is probably wise to run calculations at two > different levels of theory to distinguish between general trends and > methodological artifacts. By carefully choosing a statistically reliable > level of theory, you can at least minimise the possibilibity of making > wrong predictions, and that is something that also industry people will > accept if properly explained. I hope some of these hints were helpful. > > Finally, I hope you forgive me for saying that having efficient and ever > improving QM methods is something that would not be possible without all > those dedicated level nerds out there, which is why we should regard that > term as a compliment. > > Cheers, > Lars > > --- > Dr. Lars Goerigk > ARC DECRA Fellow > School of Chemistry > The University of Melbourne > VIC 3010 > Australia > > Research profile: > http://www.chemistry.unimelb.edu.au/dr-lars-goerigk > List of my publications: > http://www.researcherid.com/rid/D-3717-2009 > Follow me on Twitter: https://twitter.com/lgoer_compchem> > > --001a1141f37e502c9a051f5f51e9 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Hi Lars:

There is no need for = defense. We all agree that method development is important.

Persona= lly, I am a bit defensive of my own work which was published in 2009 in a n= on-theoretical journal and consist of 80% experiment. I provided that examp= le to show one where theory (whatever it is) worked well to the point of pr= ediction. Again, this is what I am looking for in this thread.

Unfo= rtunately, theory can not exist on its own and the finances flow from exper= iment downward. You will find only blank looks with the arguments you have = made outside of a room full of theorists and that is the reality. On the ot= her hand, often we are in front of chemists, at least, and having verified = chemical examples of the importance of theory in necessary. These should be= chemically understandable and should be dealing with issues that are impor= tant to the audience. They should be transferable. That means using standar= d methods with the right statistics and history that can be computed in a r= easonable time and executed, potentially, by non-experts. So, while I love = MRCI, for example, even the best of us will have trouble getting it right o= n relevant molecules.

=C2=A0Best,

Peter
=

On Wed, Sep 9, 20= 15 at 4:06 PM, Lars Goerigk lars.goerigk-,-unimelb.edu.au <owner-chemistry**ccl.net> wrote:<= br>

Sent to CCL by: "Lars=C2=A0 Goerigk" [lars.goerigk : unimelb.edu.au]
Hi Peter,

in defense of us people that believe in obtaining the right result for the = right reason: even if industry is interested in quick-and-dirty results, wo= uld it not be highly embarassing to make recommendations based on a method = that relies on unforeseeable error compensation, only to then see that the = experimentalists cannot reproduce your predictions? Susi's comment was = therefore valid, albeit I would have worded it in a more diplomatic way.
I find it very important to apply a level of theory that comes with a low r= isk of unsystematic error compensation. For example, a dispersion-corrected= (double-)hybrid density functional with at least a triple-zeta basis set i= s overall much more reliable than the popular B3LYP/6-31G* level; the reaso= ns for that are clear and they have been discussed extensively in the liter= ature and in this forum. If you cannot afford a large basis set, but you wa= nt to quickly obtain results for bigger systems with an acceptable accuracy= , there are other promising methods out there (e.g. HF-3c or PBEh-3c). More= over, it is also important to use reliable solvation models (e.g. COSMO-RS)= and reliable enthalpy and entropy corrections.

As to your comment on "level nerds" being a barrier for successfu= l interaction between theory and experiment, quantum chemists do not invest= igate and develop new methdods just because they do not know what else to d= o with their time. I would rather say that this discussion highlights a lac= k of communication between method developers and the users of QM methods an= d as always the fault for that lies probably on both sides; one consequence= of that is that B3LYP/6-31G* is still so popular in the year 2015.

Giving a general answer to your question is difficult, as it is not quite s= ure what property you want to calculate, which is exactly why the various a= nswers to your question differ so much. However, there are numerous benchma= rk studies out there that provide some guidance on which methods to use and= which to avoid; I have given some hints on reliable methods above.

Of course, there is never 100% certainty that your predictions are right be= cause in the end you try to describe a highly complex system with a theoret= ical model, which by definition is only a simplification of the real world.= That complexity also means that there is not one sole, easy solution to yo= ur question. Therefore, it is probably wise to run calculations at two diff= erent levels of theory to distinguish between general trends and methodolog= ical artifacts. By carefully choosing a statistically reliable level of the= ory, you can at least minimise the possibilibity of making wrong prediction= s, and that is something that also industry people will accept if properly = explained. I hope some of these hints were helpful.

Finally, I hope you forgive me for saying that having efficient and ever im= proving QM methods is something that would not be possible without all thos= e dedicated level nerds out there, which is why we should regard that term = as a compliment.

Cheers,
Lars

---
Dr. Lars Goerigk
ARC DECRA Fellow
School of Chemistry
The University of Melbourne
VIC 3010
Australia

Research profile:
http://www.chemistry.unimelb.edu.au/dr-lars-goeri= gk
List of my publications:
http://www.researcherid.com/rid/D-3717-2009
Follow me on Twitter: https://twitter.com/lgoer_compchem



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