Subject: MOPAC 6.0 question

From: [email-masked] (Hr Dr. S. Shapiro)
Date: Mon, 5 Aug 1996 10:14:10 +0200 (MEST)

Dear Colleagues;

     I wish to perform a FORCE calculation on a geometrically possible but not
energetically favourable rotamer of a certain small molecule.  The geometry of
this molecule was first "optimised" using the keywords PM3 NOINTER EF PRECISE,
and a gradient norm <0.01 was achieved without difficulty.  The optimised geo-
metry was then fed into MOPAC 6.0 with the optimisation flag set to "0" for all
bond lengths, bond angles, and dihedral angles (because I need to have informa-
tion on the "optimised" geometry without further modification) using the key-
words PM3 NOINTER FORCE.  The calculation went well, but I noticed that the
gradient norm for the FORCE calculation was 4.3.

     My questions are are follows:

     (i) It is my understanding that a gradient norm <0.2 is considered quite
acceptable for MOPAC calculations.  However, for FORCE calculations there is a
warning message: **GRADIENT TOO LARGE TO ALLOW FORCE MATRIX TO BE CALCULATED
(LIMIT=10)**.  Is a gradient norm = 8, 9, or 10 rather large, even for a FORCE
calculation?  If so, then why is the cut-off limit for FORCE calculations set to
such an unrealistically high value as 10, rather than, say, 1 or 2?

    (ii) For a FORCE calculation, what are the units of the gradient norm?

   (iii) As stated above, for the "optimised" geometry of my structure I obtain-
ed a gradient norm <0.01; yet, when subjected to a FORCE calculation, a differ-
ent gradient norm value (4.3) was obtained?  Why is this?  Is the gradient norm
for the geometry optimisation run (PM3 NOINTER EF PRECISE) different from the
gradient norm for the PM3 NOINTER FORCE run?

    (iv) I have heard that the keyword NLLSQ can be employed to help minimise
the gradient norm for a FORCE calculation.  However, when I used the keywords
PM3 NOINTER FORCE NLLSQ, I got an error message that MOPAC can't run with more
than one geometry option.  What is the proper set of keywords to do a FORCE cal-
culation using the PM3 parametrisation in which the gradient norm is to mini-
mised using Bartel's method?

     (v) If the keyword NLLSQ is inappropriate for the use to which I wish to
apply it, is there another strategy that I can use to help bring down the gradi-
ent norm for a FORCE calculation using a geometry-fixed structure (optimisation
flags all set equal to 0) with the PM3 parametrisation?

    (vi) In a FORCE calculation, so long as the gradient norm is below the cut-
off limit of 10, does it _really_ matter just what the value of the gradient
norm is?  I ought to mention that for the _most favoured_ rotamer also optimised
using the keywords PM3 NOINTER EF PRECISE (resulting gradient norm <0.01), the
corresponding FORCE calculation (keywords PM* NOINTER FORCE) leads to a gradient
norm of 0.35.

     Thanks in advance to all responders.

Sincerely,

S. Shapiro
ZH, CH
[email-masked]