Penfold L7 Flashcards
(22 cards)
What is a basis set
set of function that can be used to describe every type of wavefunction
What do we mean when we say more flexibility
increase the number of basis function
Describe what happens if we increase the number of basis function
The more basis functions we have the more likely we are to do a better job of representing the molecular wave function. Therefore if we have a better wavefunction we will have a more accurate energy
What is a slater type orbital in terms of basis functions
A Slater-Type Orbital is a realistic atomic-like function used in quantum chemistry basis sets.
It has an exponential shape, closely resembling actual atomic orbitals
What is the problem with slater type orbitals
when we go from atomic orbital to molecular orbital we are combining a lot of slater type orbitals and the maths associated with overlapping and combining these is very complicated
What is the most common basis functions used in quantum chemistry calculation
gaussian type orbitals (GTOs)
What are the benefits of using Gaussian type orbitals
the product of two gaussians centred on different atoms is another Gaussian centred between the atoms. The maths for gaussians is also a lot easier
What are the limitations to using gaussians
Gaussian decay drops off too fast compared to real atomic orbitals
How do we fix the limitations surrounding using gaussians
we combine multiple gaussians
What are the two different types of gaussians
uncontracted/ primitive gaussians
Contracted gaussians
What is a uncontracted/primitive Gaussian
coefficient forming the basis set of atomic orbitals are also being optimised and are allowed to vary during the calculation
What are contracted gaussians
coefficients are fixed - they have been pre-computed and are not optimised during the calculations
What is STO-3G
STO - slater type orbital
3G - 3 gaussians
Meaning each slater type orbitals are expanded using 3 gaussian functions
Describe the STO-3G basis set
basis set is fixed and contracted - coefficients not changed during calculations. STO-3G is an example of a minimal basis set
Define minimal basis set
each atomic orbitals is described by 1 set of gaussians
How can we improve accuracy for STO-3G
we could use more gaussians function per atomic orbitals but STO-4G uses 4 gaussians functions per atomic orbital but results do not improve much and this is rarely used
What is the problem with STO-3G
since the radial exponents of the basis set are pre-computed the atomic orbitals cannot distort in response to their molecular environment
What is a split valence basis set
A split-valence basis set is a type of basis set where valence orbitals are described using more than one basis function, while the core orbitals are described with fewer.
What is 6-31G
6-31G is a commonly used split valence basis set. One function per core state and two per valence state
What is 6-311G basis set
split valence, one function per core state and three per valence state
What is 6-31+G
6-31G plus one diffuse function per valence state on heavy atoms
What is 6-31G*
6-31G plus 6 polarisation functions
