Vibrational Analysis (IR
Spectroscopy from DFT)
“After optimizing
a molecule using DFT, how do we know the structure
is correct and stable?”
Then say:
“We verify
it using vibrational analysis and compare with IR spectra.”
What is Vibrational Analysis?
Definition:
Vibrational analysis studies how atoms in a molecule move (vibrate) around their equilibrium positions.
Time-Dependent Density Functional Theory (TD-DFT)
“What
does DFT calculate — ground state or excited state?”
Answer: Ground
state
Then say:
But what happens when a molecule
absorbs light?
An electron gets excited.
DFT cannot properly describe this — so we need TD-DFT.
Basis
Sets in Quantum Chemistry
HF, MP2, and CC are methods used for how to calculate
energy
Now the question is
With what mathematical tools do we represent orbitals? That tool is called Basis Sets
Hartree–Fock (HF) Method
Scientists
asked:
“How
can we simplify the many-electron problem?”
Replace
complex interactions with an average field. This is the Hartree–Fock idea.
Each electron moves in the average field of all other electrons instead of tracking exact electron–electron motion.
Coupled Cluster (CC)
is one of the most accurate methods for calculating electronic structure.
It systematically includes electron correlation and is widely used in quantum chemistry.
Møller–Plesset Perturbation Theory (MP2)
So we need a method that is
Electron Correlation
& Configuration Interaction (CI)
Do electrons in a molecule really move independently?
Answer:
No
They repel each other and try to
avoid each other all the time.
In Hartree–Fock
(SCF):
