Question

Compared to experimental values, are the CIS excitation energies generally overestimated or underestimated? Explain why..

Taking the ground state of beryllium atom as an example, give an example of singly, doubly, triply and quadruply excited determinants. Within correlated methods, which type of excitations is the most important? Explain why single excitations are usually unimportant.

Answer 1

Compared to experimental values, the configuration interaction singles (CIS) excitation energies are generally overestimated.

Explanation: Other computationally efficient methods, such as CIS, 57,58-time dependent DFT (TDDFT) and ∆SCF have been well-known to describe excitation energies with success, meanwhile they have an important weakness. Particularly, CIS can overestimate excitation energy by 2 eV.