Potential Energy Surfaces (PES)

Question 1

How can we refer to the ground-state potential energy surface of a system?

Answer 1

The function V(R) where R is a vector containing the coordinates of all atomic nuclei in a system and V is the ground state electronic energy for a system.

Question 2

What are degrees of freedom and how many can be present within a system?

Answer 2

A degree of freedom is a variable of the function V(R). V can have up to 3N-6 degrees of freedom (where N is the number of atoms present) because each atom can move in the x, y and z directions but three rotations and three translations of the whole system have no effect on V and therefore are not considered degrees of freedom in practice (except for diatomic molecules where 3N-5=1). V can only be plotted against q1 and q2 at once.