Home
Computational Chemistry
Overview Materials
Computational Science
Computational Chemistry
Basic Quantum Chemistry
Schrodinger's Equation
Atomic Units
The Born Oppenheimer Approximation
The Hartree Fock Approximation
Key Points
Readings
Overview
Atomic Orbitals
Lab Activities
Zmatrices
Basis Sets
Geometry Optimizations
Ionization Energies
Support Materials
Interactive Tools
Glossary of Terms
Quick Guide to DISCO Output File
Related Links
ChemViz
Computational Chemistry
SUCCEED's Computational Chemistry Developers' Tools
What's New?
Discussion Board
Team Members
Email the Group
Contact Webmaster

How Much Do I Already Know?
Key Points
Overview
Chemists have been some of the most active and innovative participants in this
rapid expansion of computational science. Computational chemistry is simply
the application of chemical, mathematical and computing skills to the solution
of interesting chemical problems. It uses computers to generate information
such as properties of molecules or simulated experimental results. Some common
computer software used for computational chemistry includes:
 Gaussian xx, Gaussian 94 currently
 GAMESS
 MOPAC
 Spartan
 Sybyl
Computational chemistry has become a useful way to investigate materials that
are too difficult to find or too expensive to purchase. It also helps chemists
make predictions before running the actual experiments so that they can be
better prepared for making observations.
The Schroedinger equation (explained in another section) is the basis for most
of the computational chemistry scientists use. This is because the
Schroedinger equation models the atoms and molecules with mathematics. For
instance, you can calculate:
 electronic structure determinations
 geometry optimizations
 frequency calculations
 transition structures
 protein calculations, i.e. docking
 electron and charge distributions
 potential energy surfaces (PES)
 rate constants for chemical reactions (kinetics)
 thermodynamic calculations heat of reactions, energy of activation
Currently, there are two ways to approach chemistry problems: computational
quantum chemistry and noncomputational quantum chemistry
Computational quantum chemistry is primarily concerned with the numerical
computation of molecular electronic structures by ab initio and
semiempirical techniques and noncomputational quantum chemistry deals with
the formulation of analytical expressions for the properties of molecules and
their reactions.
We just mentioned ab initio and semiempirical numerical techniques.
Definitions of these terms are helpful in understanding the use of
computational techniques for chemistry. Scientists mainly use three different
methods to make calculations:
 ab initio, (Latin for "from scratch") a group of methods in which
molecular structures can be calculated using nothing but the Schroedinger
equation, the values of the fundamental constants and the atomic numbers of
the atoms present (Atkins, 1991).
 Semiempirical techniques use approximations from empirical (experimental)
data to provide the input into the mathematical models.
 Molecular mechanics uses classical physics to explain and interpret the
behavior of atoms and molecules
The table below attempts to capture the specifics of each of these three methods:
Method Type 
Advantages 
Disadvantages 
Best for 
Molecular Mechanics

 uses classical physics
 relies on forcefield with embedded empirical parameters

 Computationally least intensive  fast and useful with limited computer resources
 can be used for molecules as large as enzymes

 particular force field applicable only for a limited class of molecules
 does not calculate electronic properties
 requires experimental data (or data from ab initio) for parameters

 large systems (thousands of atoms)
 systems or processes with no breaking or forming of bonds

SemiEmpirical

 uses quantum physics
 uses experimentally derived empirical parameters
 uses approximation extensively

 less demanding computationally than ab initio methods
 capable of calculating transition states and excited states

 requires experimental data (or data from ab initio) for parameters
 less rigorous than ab initio) methods

 mediumsized systems (hundreds of atoms)
 systems involving electronic transitions

Ab Initio

 uses quantum physics
 mathematically rigorous, no empirical parameters
 uses approximation extensively

 useful for a broad range of systems
 does not depend on experimental data
 capable of calculating transition states and excited states

 computationally expensive

 small systems (tens of atoms)
 systems involving electronic transitions
 molecules or systems without available experimental data ("new" chemistry)
 systems requiring rigorous accuracy

To summarize, computational chemistry is:
 a branch of chemistry that generates data which complements experimental
data on the structures, properties and reactions of substances. The
calculations are based primarily on Schroedinger's equation and include:
 calculation of electron and charge distributions
 molecular geometry in ground and excited states
 potential energy surfaces
 rate constants for elementary reactions
 details of the dynamics of molecular collisions
 particularly useful for:
 determination of properties that are inaccessible experimentally
 interpretation of experimental data
