Subject: MOPAC question

From: "Hr. Dr. S. Shapiro" <[email-masked]>
Date: Thu, 14 Jan 1999 09:14:53 +0100

Dear Colleagues;

	I have a couple of questions about the internal workings of the MOPAC
optimisation algorithms: 

	Consider a large molecule A-B, where A and B are separate portions of the
molecule connected to one another by a single sigma bond (C-C, C-O, or
C-N).  The units A and B have been optimsed in terms of bond lengths,
angles, and dihedral angles, but the A-B bond is not optimal (and in fact
may be rather far from optimal).  Now suppose you do a MOPAC run with all
optimisation flags set on "1" and the keywords "am1 ef precise".  

(a) Will MOPAC recognise that the two moieties A and B are already
optimised and so more-or-less leave their "internal structures" alone
whilst focussing its attention on optimising the A-B link, or will it
"deoptimise" and then "reoptimise" the whole structure?

(b) Suppose that instead of all optimisation flags being set to "1" those
for moiety A are set to "0" whilst those for moiety B and for the A-B link
are set to "1".  Will MOPAC "drag" the intact optimised moiety B over to A
in the course of optimising the A-B link, or will it "deoptimise" and then
"reoptimise" the whole B moiety?

Thanks in advance to all responders,

S. Shapiro
[email-masked]