What does LCAO stand for, and why is it used in computational chemistry? State t

Question

What does LCAO stand for, and why is it used in computational chemistry? State the reason why the Hartree-Fock equations for poly-atomic molecules are solved via basis set expansions for the orbital's What is the reason for using 'split-valence', 'triple- ', 'quadruple- ', ate. basis sets in molecular calculations? Would it be possible to perform an accurate calculation of the H_2 molecule by placing basis functions only on one of the hydrogen? Give reasons for your answer. What is a 'minimal basis' ? Explain why an MO calculation for H_2, say, should not use only 1s basis functions but also 2p, 3d, etc. What is the difference between STO and GTO basis functions? Why do we typically need many more GTO functions than STOs for calculations of similar accuracy?

Explanation / Answer

a) LCAO is a method used in cuantum chemistry in order to calculate molecular orbitals in quantum chemistry. This is used in computational chemistry because it allows to predict wether an orbital can mix in order to form molecular orbitals and how many molecular orbital can be formed.

b) Hartree-Fock equations can be solved by a basis set because it is an iterative process and it is easier for a computational software to solve it rather than a human.

c) Because we need to increase the size of the basis set. One way to increase the size of a basis set is to take more basis functions per atom. Split valence basis sets, such as 3-21G and 6-31G basis sets, have two (or more) sizes of basis function for each valence orbital.

e) A minimal basis set is a set which contain the minimum number of basis functions that are needed for each atom

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