Restricted calculation use single orbital for a pair of electrons. One electron with up and another electron with down spin occupy the same orbital. In unrestricted calculations, each electron occupy different orbital.
Okay. Which one to use for the calculations?
Generally, default formalism for closed-shell system is restricted calculation and the for the open-shell molecule, default calculation is unrestricted.
But, it is essential to do unrestricted calculations on closed-shell calculations also to confirm the ground state. For radicals (diradicals and polyradicals), it is essential to use both restricted and unrestricted calculations.
The restricted and unrestricted calculations should yield the same energy which implies there no symmetry breaking occur in those system.
The typical example for a molecule with triplet ground state is Oxygen molecule.
Read more on the following links:
Okay. Which one to use for the calculations?
Generally, default formalism for closed-shell system is restricted calculation and the for the open-shell molecule, default calculation is unrestricted.
But, it is essential to do unrestricted calculations on closed-shell calculations also to confirm the ground state. For radicals (diradicals and polyradicals), it is essential to use both restricted and unrestricted calculations.
The restricted and unrestricted calculations should yield the same energy which implies there no symmetry breaking occur in those system.
The typical example for a molecule with triplet ground state is Oxygen molecule.
Read more on the following links:
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