Subject: Restraining dihedrals using HyperChem

From: "M�rcio Cyrillo" <[email-masked]>
Date: Mon, 21 Sep 1998 15:41:48 -0300

Dear HyperChemists,

I would like to ask you something that I think is very strange about
HyperChem Geometry Optimization (GO) calculations. Actually I had
already asked this before but the latter support man from HyperCube (Dr.
Stravrev) did not want or did not know how to answer to my question. I
think its about time to clear this issue! I hope someone can help me.
The problem is: we read in HyperChem manual that for GO optimizations we
can restraint a variable (bond length, angle or dihedral) using a
restraing constant that adds a force field to the hamiltonian of the
system so that a potential like that of a spring can "hold" the variable
to a specific supplied value. This "Hook" constant, default value equals
60,  is set up depending on the problem, but in the manual they tell us
that it cannot be as big as we want because we would then add a
nonlinear term that would perturbate the original hamiltonian producing
artificial results.
I have the following questions:
- For molecular mechanics I understand this procedure of adding a
quadratic potential to the hamiltonian to restrain a variable, but it
seems to me as being meaningless to apply the same procedure to a
semi-empirical hamiltonian. Am I wrong?
- In mopac we can easily accomplish the restraint of a variable -
changing a digit from 1 to 0 in the z-matrix scheme. Why did not
HyperChem use the same algorithm? With MOPAC it works fine, which is
pretty different in HyperChem where it almost impossible to, in fact,
restrain the variable to the desired value. Everyone that has used my
program HyperSpin (www.ifi.unicamp.br) before, knows what I mean.
Talking about HyperSpin, I am going to release a new version soon but I
am also working on a interface that will do quite the same work but
using old and good MOPAC6.
I am looking forward to hearing from you soon, best regards,
Marcio.
--
Marcio Cyrillo - http://www.ifi.unicamp.br/~cyrillo
email: [email-masked] or [email-masked]
ICQ: 14059279
Graduate Student
State University of Campinas (UNICAMP), SP, Brazil
Applied Physics Department - DFA/IFGW
room 51, phone +55 19 788 5364, fax +55 19 788 5376