tag:blogger.com,1999:blog-5439168179960787195.post1378303859517649450..comments2020-01-24T23:17:58.637+10:00Comments on Condensed concepts: Overselling ab initio computational chemistryRoss H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-5439168179960787195.post-84514978954708859912012-07-19T11:50:19.960+10:002012-07-19T11:50:19.960+10:00What do you mean by slow electronic variables? Slo...What do you mean by slow electronic variables? Slow with regards to what?Unknownhttps://www.blogger.com/profile/02072594070501336602noreply@blogger.comtag:blogger.com,1999:blog-5439168179960787195.post-24024004090364035862012-07-06T09:07:31.395+10:002012-07-06T09:07:31.395+10:00I think this highlights the point that the best us...I think this highlights the point that the best use of "ab initio" techniques (which are almost ALWAYS based on a self-consistent field) is to identify relevant observables that can be used to produce reduced models. Scaling CASSCF energies is a common strategy; in this case the point is that the self-consistent field is identifying a compact set of "slow" electronic variables that can be used in a reduced state space. The only difference between this and model building is that in the latter case, one asserts the identity of the slow variables based on other means (i.e. deep thought and physical intuition). The methods can be used in tandem; one can use self-consistent fields to support models if it can be shown that the model states correspond to a self-consistent field solution. This supports the model by showing that the relevant variables correspond to "slow" variables as identified by an objective method. Of course, if this fails then it is not a priori true that the "ab initio" approach is better, because self-consistent fields will embody any errors in the Hamiltonian. In this particular case, the errors in the Hamiltonian would include the fact that there is only a cluster model being used to represent a macroscopic lattice, that the clamped-nucleus approximation is invoked, and that the electron correlation treatment may not be appropriate for the physics of the system.Seth Olsenhttps://www.blogger.com/profile/09304457461800104790noreply@blogger.com