Relating Basis Set, Number of Basis Functions, and CPU Time

Background

In computational chemistry, the calculations which are performed involve very complex mathematics. The level of theory chosen indicates how much knowledge about a given molecule you are assuming; ab initio, the level we will use, assumes no prior knowledge about the molecule. All data is the result of solving the equations describing the molecule. The exact equations describing these molecules cannot be solved exactly, so approximations are created and then solved. The "basis set" describes the approximation to the exact equations of the molecule. These sets are a starting guess, or initial idea, about the molecule in question.

Basis sets usually attempt to approximate the exact equations by adding up several other curves, called basis functions. The basis set chosen describes the number of basis functions used as well as the way they are added up. The name of a basis set is based on the number of basis functions used to represent different features of the molecule.

When working at the ab initio level of theory for computational quantum chemistry, the choice of basis set is important. This affects the accuracy of your results as well as the computational expense associated with any results. A compromise must be reached between accuracy and computational time, since the time involved can be large, even on the best of computers.

Behind the issue of computation time is the idea of basis functions. The number of functions being used in a particular computation is directly related to the computation time required. The choice of basis set determines how many functions (ie, gaussian curves) will be used to express the molecule, depending on the number of atoms of each type present. In this activity, you will determine the relationship between basis set, number of atoms, and number of functions, and the relationship between number of functions and CPU time for several alkanes.


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http://shodor.org/succeed-1.0/programs/compchem98/labs/settime/