Chem3D Users Manual - CambridgeSoft

Appendix J: MOPAC
Overview
The appendix contains miscellaneous information
about MOPAC.
You can find additional information about
MOPAC by visiting the MOPAC home page at:
http://www.cachesoftware.com/mopac/index.shtml
MOPAC Background
MOPAC was created by Dr. James Stewart at the
University of Texas in the 1980s. It implements
semi-empirical methodologies for analyzing
molecular models. (MOPAC stands for Molecular
Orbital PACkage.) Due to its complexity and
command line user interface, its use was limited
until the mid 1990s.
Since version 3.5 (1996), Chem3D has provided an
easy-to-use GUI interface for MOPAC that makes
it accessible to the novice molecular modeller, as
well as providing greater usability for the veteran
modeller. We are currently supporting MOPAC
2000.
MOPAC 2000 is copyrighted by Fujitsu, Ltd.CS
MOPAC is the licensed version that runs under
Chem3D.
Potential Functions
Parameters
MOPAC provides five potential energy functions:
MINDO/3, MNDO, PM3, AM1, and MNDO-d.
All are SCF (Self Consistent Field) methods. Each
function represents an approximation in the
mathematics for solving the Electronic Schrödinger
equation for a molecule.
Historically, these approximations were made to
allow ab initio calculations to be within the reach of
available computer technology. Currently, ab initio
methods for small molecules are within the reach of
desktop computers. Larger molecules, however, are
still more efficiently modeled on the desktop using
semi-empirical or molecular mechanics
methodologies.
To understand the place that the potential energy
functions in MOPAC take in the semi-empirical
arena, here is a brief chronology of the
approximations that comprise the semi-empirical
methods. The first approximation was termed
CNDO for Complete Neglect of Differential
Overlap. The next approximation was termed
INDO for Intermediate Neglect of Differential
Overlap, Next followed MINDO/3, which stands
for “Modified Intermediate Neglect of Differential
Overlap”. Next was MNDO, which is short for
“Modified Neglect of Differential Overlap” which
corrected MINDO/3 for various organic
molecules made up from elements in rows 1 and 2
of the periodic table. AM1 improved upon MNDO
markedly. Finally the most recent, PM3 is a
reparameterization of AM1. The approximations in
PM3 are the same as AM1.
This sequence of potential energy functions
represents a series of improvements to support the
initial assumption that complete neglect of diatomic
orbitals would yield useful data when molecules
proved too resource intensive for ab initio methods.
Appendices
ChemOffice 2005/Appendix MOPAC • 293
Potential Functions Parameters