# 12: Group Theory - The Exploitation of Symmetry

Group Theory is a branch of the mathematical field of algebra. One important application, the theory of symmetry groups, is a powerful tool for the prediction of physical properties of molecules and crystals. It is for example possible to determine whether a molecule can have a dipole moment. Many important predictions of spectroscopic experiments (optical, IR or Raman) can be made purely by group theoretical considerations. The qualitative properties of molecular orbitals can be obtained from group theory (whereas their precise energetics and ordering have to be determined by a quantum chemical method). In quantum chemistry, group theory can applied to ab initio or semi-empirical calculations to significantly reduce the computational cost.

• 12.1: The Exploitation of Symmetry
Symmetry can be used to simplify calculations.
• 12.2: Symmetry Elements
A symmetry operation is an action that leaves an object looking the same after it has been carried out. Each symmetry operation has a corresponding symmetry element, which is the axis, plane, line or point with respect to which the symmetry operation is carried out. The symmetry element consists of all the points that stay in the same place when the symmetry operation is performed.
• 12.3: Symmetry Operations Define Groups
A mathematical group is defined as a set of elements ($$g_1$$, $$g_2$$, $$g_3$$...) together with a rule for forming combinations $$g_j$$. The number of elements $$h$$ is called the order of the group. For our purposes, the elements are the symmetry operations of a molecule and the rule for combining them is the sequential application of symmetry operations investigated in the previous section.
• 12.4: Symmetry Operations as Matrices
Matrices can be used to map one set of coordinates or functions onto another set. Matrices used for this purpose are called transformation matrices. In group theory, we can use transformation matrices to carry out the various symmetry operations discussed previously. As a simple example, we will investigate the matrices we would use to carry out some of these symmetry operations on a vector in 2D space (x,y) .
• 12.5: The $$C_{3V}$$ Point Group
• 12.6: Character Tables
Now that we’ve learnt how to create a matrix representation of a point group within a given basis, we will move on to look at some of the properties that make these representations so powerful in the treatment of molecular symmetry.
• 12.7: Characters of Irreducible Representations
the character of a group representation is a function on the group that associates to each group element the trace of the corresponding matrix. The character carries the essential information about the representation in a more condensed form.
• 12.8: Using Symmetry to Solve Secular Determinants
As we continue with this course, we will discover that there are many times when we would like to know whether a particular integral is necessarily zero, or whether there is a chance that it may be non-zero. We can often use group theory to differentiate these two cases. You will have already used symmetry properties of functions to determine whether or not a one-dimensional integral is zero. For example, cos(x) is an ‘even’ function (symmetric with respect to reflection through the origin),
• 12.9: Generating Operators
• 12.E: Group Theory - The Exploitation of Symmetry (Exercises)
These are homework exercises to accompany Chapter 12 of McQuarrie and Simon's "Physical Chemistry: A Molecular Approach" Textmap.