afterwardswe will likely check the method against some experimental facts). So it
would appear that literally the SE method, parameterized by ab initio or DFT, is not
really semiempirical; however, it is still in the spirit of SE methods, circumventing
detailed calculation of the Fock matrix elements (using pre-calculated values!).
Reference
- Lewars E (2008) Modeling marvels. Computational anticipation of novel molecules. Springer,
The Netherlands
Chapter 6, Harder Questions, Answers
Q4
There is a kind of contradiction in the Dewar-type methods (AM1, etc.) in that
overlap integrals are calculated and used to help evaluate the Fock matrix elements,
yet the overlap matrix is taken as a unit matrix as far as diagonalization of the Fock
matrix goes. Discuss.
In the simple H€uckel method, which is not a Dewar-type method, the use of
overlap integrals as the sole source of geometric (connectivity) information is
transparent. In AM1 and its relative PM3, which are modified versions of MNDO,
overlap integrals are also calculated, and used in a somewhat more involved way to
evaluate some of the core integrals. Yet after assembling the Fock matrix this is
simply diagonalized to give coefficients and energies (repeatedly, in the SCF proce-
dure) without using orthogonalization to alter the original Fock matrix or to “reset”
the coefficients. The sidestepping of orthogonalization in the SHM is achieved by
setting the overlap matrix equal to a unit matrix, i.e. by simply setting allSii¼1 and
allSij(i 6 ¼j)¼0. This is a logical inconsistency, but it works quite well!
Chapter 6, Harder Questions, Answers
Q5
What would be the advantages and disadvantages of using the general MNDO/AM1
parameterization procedure, but employing a minimal basis set instead of a minimal
valence basis set?
A minimal basis set is bigger than a minimalvalencebasis set by the inclusion of
core atomic orbitals, e.g. a 1s AO for carbon, and 1s, 2s, and three 2p AOs for
silicon. Including these in the electronic calculation probably should not lead to
Answers 631