Question

what is the difference between calculated (gaussia view) and experimental IR spectrum?

Answer 1

Gaussian

It is an electronic structure package capable of predicting many properties of atoms, molecules, and reactive systems Including

1. Energy  
2. Structure
3. Vibration frequency

utilizing ab initio, density functional theory, semi-empirical,molecular mechanics, and hybrid methods.

The level of theory is the set of underlying approximations used to describe the chemical system.

Semi-empiricalAM1, PM3,.. etc.

Density functional theory B3LYP, MPW1PW91, …etc

AND basic set are used calculation of Gaussian i,e wave function of electron .

example Prople -3-21G

ON OTHER HAND IR can only tell us about the types of functional group present in the molecule it does not tells about energy, structure .