From: kenneth@davy.uio.no (Kenneth Ruud)
Newsgroups: sci.chem
Subject: Re: Mollecular Modelling
Date: 19 Dec 92 18:07:48 GMT
Organization: University of Oslo, Norway

In article <1gv8c7EINN8b2@uni-erlangen.de> bco100@cd4680fs.rrze.uni-erlangen.de (N. van Eikema Hommes) writes:
>mulman@hmcvax.claremont.edu (Mike Ulman) writes:
>
>>Hi!
>>Can someone explain to me the difference between Ab Initio and Semi-Empirical
>>modelling? 
>>The way I understand it: the former uses the exact Hamiltonian and an approx.
>>wave equation while the latter uses an approximate Hamiltinian (PM3, etc) and
>>an exact wave equation. What does that mean? Which is recommended for Cope
>>Rearrangements and why?
>>Also, what is a singlet, doublet and triplet state, as in multiplicity 1, 2, 3.
>
>Both ab-initio and semi-empirical methods use many approximations and give you
>approximate wave functions. Semi-empirical methods are much more approximate,
>but compensate this by the use of parameters, which are optimized to reproduce
>experimental data.
>A good introductory textbook is "A Handbook of Computational Chemistry"
>by T. Clark (Wiley-Interscience, 1985). It contains a description of the
>various computational methods (Molecular Mechanics, Semi-empirical and
>Ab-Initio), as well as a general introduction into quantum mechanics.
>For the newer methods (AM1, PM3), refer to the original literature (AM1:
>M.J.S.Dewar, JACS, 1985; PM3: J.J.P.Stewart, J.Comp.Chem. 1988 or so, don't
>have the references available at the moment).
>Ken Houk just published a review on pericyclic reactions in Angew.Chem., a few
>months ago. Much computational stuff, should be worth reading.
>What is recommended largely depends on what your computational facilities are.
>If you have problems keeping your private Cray YMP-90 busy, then ab-initio is
>the method of choice. Otherwise, semi-empirical methods are at least a good
>start. AM1 should do well.
>
Well, I would say that it is a bit of an exaggeration to say that you have to
have a personal Cray YMP-90 with lot of spare time before doing ab-initio
calculations. Of course, ab-initio calculations is not recommended on a PC, but
otherwise a see no big problem connected with doing such calculation, unless
you have big molecules.

You should however think of what you want to find out from these calculations.
As Dr.van Eikema Hommes writes, semi-empirical calculations are based upon
some given parameters optimized with respect to experimental results on some
molecules. This makes semi-empirical methods very powerful when studying 
molecules very similar in structure to those molecules the parameters are 
optimized against.

This, however, also puts a big contraint on the applications of these programs,
in the way that they cannot predict anything on molecules which differ a lot
in structure from the test molecules. You then have to alternatives:

- Introduce new parameters yourself so that also these structures can be
handled

- Turn to ab-initio methods

Ab-initio means "from the start", that is, we start with the physics and assume
the quantum mechanical laws and equations to be true, and then try to solve 
them for different kind of molecules and properties without ever involving
experimental parameters. However, it is not possible to solve these equations 
exact, and so it has been developed several approximate, iterative schemes to 
solve these problems. These schemes are however often very time and disc 
consuming, but gives nice results if used properly.

To the second question concerning multiplicity, you are probably familiar with
electron spin. In a molecule you have in most cases several electrons all of
which have a spin. You can then add the spins in a vectorial manner to get a
total electron-spin, S, for the molecule. You then have 2S+1 different 
components of this total spin, all of which has the same energy (are 
degenerate). This number, 2S+1, is called the spin-multiplicity of the 
molecule.
>
>All the best,
>
>    Nico van Eikema Hommes
>--
>+======================================+=====================================+
>|  Dr. N.J.R. van Eikema Hommes        |  hommes@organik.uni-erlangen.de     |
>|  Institut fuer Organische Chemie I   |  Tel. : 49/0 - 9131 - 85 - 4096     |
>|  Henkestr. 42, D-8520  Erlangen, FRG |  Fax  :                  - 9132     |
>+======================================+=====================================+
>

Hopes this can be of some help!

Kenneth

--
_______________________________________________________________________________
Kenneth Ruud, doing Chemical Physics at the Department of Chemistry,
University of Oslo, Norway.         E-mail: kenneth@dalton.uio.no

I don't know what the computer language of the year 2000 will look like, but
I know it will be called FORTRAN.
_______________________________________________________________________________
Newsgroups: sci.chem