ChemNews.Com VOL 6 NO 4
New Chemistry with Gaussian 98
AEleen Frisch, PH.D.
Gaussian 98 is the latest in the Gaussian series of
electronic structure programs. It is designed to model a broad range
of molecular systems under a variety of condition. For example, experimental
chemists can use it to study molecules and reactions of definite or
potential interest, including both stable species and those compounds
which are difficult or impossible to observe experimentally (short-lived
intermediates, transition structures and so on).
New Features, New Chemistry
Gaussian 98 can predict the energies, molecular structures,
vibrational frequencies and numerous molecular properties for systems
in the gas phase and in solution, and it can model both their ground
state and excited states. Chemists can apply these fundamental results
to their own investigations, using it to explore chemical phenomena
like substituent effects, reaction mechanisms and electronic transitions.
Gaussian 98 includes many features specifically designed
to bring large molecular systems within reach of electronic structure
methods. Advanced optimization algorithms and other efficiency innovations
make semi-empirical geometry optimizations practical for larger
molecules than ever before.
Fast multipole method (FMM) and sparse matrix techniques for linearizing
computational cost and an enhanced integration algorithm substantially
extend the practical range of DFT calculations, especially for frequency
calculations.
ONIOM Calculations
The ONIOM facility in Gaussian 98 brings the powerful method
of Morokuma and coworkers to Gaussian users. Using this technique,
which divides a molecular system into two or three "layers" which
are treated at different levels of accuracy, very large molecules
of biological and commercial interest become feasible for study. ONIOM
allows, for example, the active site of a drug molecule to be modeled
using a very accurate model chemistry, while atoms close to it are
treated at a somewhat lower level of theory, and atoms far from the
site are treated in a more approximate manner.
The illustration on this page indicates conceptually how a molecule
might be so divided: here the high layer, which will be modeled
most accurately, is rendered using a ball-and-stick format, the
middle layer is rendered using thick tubes, and the low layer, which
will be treated with a less expensive computational method, is rendered
in a wireframe mode.
ONIOM calculations in Gaussian 98 may use any of the available
methods for the various layers defined for the molecular system
currently under investigation.
Molecular Properties
Gaussian 98 continues to extend the range of molecular properties
that can be predicted. NMR shielding tensors and chemical shifts can
be computed at the MP2 level as well as the Hartree-Fock and DFT methods.
Vibrational circular dichroism (VCD) intensities may be predicted
at the Hartree-Fock and DFT levels. Raman intensities may be predicted
using DFT and MP2 methods in addition to Hartree-Fock.
Modeling Excited States
Gaussian 98 includes a several important methods for modeling
systems in their excited states which allow such calculations to be
performed for a wide range of molecular systems.
The ZINDO semi-empirical method is available for studying the excited
states of very large molecules. The CI-Singles method remains Gaussian
98's first-level ab initio facility for modeling excited states.
Gaussian 98 can also predict excited state energies via
time-dependent methods (TD). These calculations may be carried out
at the Hartree-Fock level or using a DFT method in order to include
some of the effects of electron correlation in modeling the excited
state.
Gaussian 98's implementation of this facility is very efficient,
allowing the latter to be computed as lower computational cost than
other excited state methods of comparable accuracy. |
