CCL: Case Studies of QM Computational Chemistry in Reactivity



 Sent to CCL by: Jerome Kieffer [Jerome.Kieffer*o*terre-adelie.org]
 On Thu, 10 Sep 2015 07:26:41 +0100
 "Peter Jarowski peterjarowski===gmail.com"
 <owner-chemistry||ccl.net> wrote:
 > I ask a simple question and further ask everyone to pretend they are at an
 > interview outside of an academic environment where you will find willing
 > and interested parties who enjoy the alphabet soup of DFT. what example and
 > what statistics would you present to justify your work. Clearly, you are
 > talking to experimentalists, as theory does not exist without them.
 >
 > How does theory drive experiment?
 > How much money does it save?
 > How much revenue can it produce?
 > How reliable is it?
 > How essential is it and what can it do that experiment can not?
 Hi all,
 I had the opportunity to do computational chemistry as part of a
 (large) industrial group, a decade ago.
 Such questions were often asked and it was not easy to be honest.
 When speaking of reactivity, everything is about the energy barrier
 between two (or more) path which leads to different products.
 Some critical steps may be mono, the other bi-molecular, so it is
 harder to compare them...
 Then comes the enthalpic and entropic contribution, not speaking about
 solvatation for spices that do not exist per-se as they are transition
 states.
 I left this job a while ago and I have to admit the "correctness" of
 most result I obtained at that time were mainly "lucky error
 cancelation":
 Concidering :
 A --> B
 A --> C
 in kinetic condition, let AB and AC be the transition states:
 [C]/[B] = exp(-(G[AC*]-G[AB*])/RT)
 The error in the exponential is 2x the error in calculating the free
 energy of one TS, one ends with a huge error bars which is anything but
 conclusive at that time (one error was about 6kcal/mol ... while with
 ony 1kcal/mol error, no conclusion was possible) .
 Cheers,
 --
 Jérôme Kieffer