Biomedical Engineering Reference
In-Depth Information
The first six primitive GTOs with exponents 1805.0 through 0.2558 contribute mostly to
what we call 'the' atomic 1s orbital. The two most diffuse s functions (those with exponents
0.7736 and 0.2558) are the main components of what we call the 2s STO, and they are
allowed to vary freely in molecular calculations. The 1s primitive with exponent 2.077
turns out to make substantial contributions to both the atomic 1s and 2s orbitals, so that
one is left free as a separate basis function.
A typical package such as Gaussian98 will have very many basis sets as part of their
database; you do not have to rediscover the wheel. On the other hand, some basis sets are
good for one particular application and some are poor and there are implications of cost.
The larger the basis set the higher the cost of the calculation, and the proportionality is far
from linear. Choice of basis set is a specialist subject, just like many others in our subject;
you simply have to take advice and look up recent literature citations.
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