# How to classify ionic and covalent excited-states?

In spectroscopy it is common to describe excited-states as ionic or covalent.

I understand the concept on a toy model e.g. $$\ce{H2}$$ with a minimal basis set

\begin{align} \Phi_0 &= [\chi_A(1) + \chi_B(1)][\chi_A(2) + \chi_B(2)]\tag{1} \\ &= \underbrace{\chi_A(1)\chi_A(2) + \chi_B(1)\chi_B(2)}_{\mathrm{Ionic \ Part}} + \underbrace{\chi_A(1)\chi_B(2) + \chi_B(1)\chi_A(2)}_{\mathrm{Covalent \ Part}}\tag{2} \end{align} $$\chi$$ are the AOs, $$A,B$$ are the atom labels, and $$1,2$$ are the electron labels.

However, how can I determine ionic/covalent for more complex molecules using computations?

For example, I am doing complete-active space self-consistent field calculations and I have the CI coefficients $$c_i$$ for the different determinants e.g. here are the $$c_i$$ coefficients for the first excited-state

    0.70266226570264  X54 A55 B56
0.70266226570264  X54 B55 A56
0.07890494054450  A54 B55 X56
0.07890494054450  B54 A55 X56


$$A$$ refers to an $$\alpha$$ electron, B a $$\beta$$ electron, and X doubly occupied orbital.

I've determined using symmetry that the first excited-state is of $$B_u$$ symmetry (orbital 55=$$b_g$$, orbital 56=$$a_u$$, $$b_g\times a_u=B_u$$).

• In Eqs. 1-2 the molecular wavefunction seems to be written as a linear combination of atomic orbitals, with every coefficient = 1? Apr 25, 2021 at 2:54
• Did you wind up finding a solution to this?
– Tyberius
Jun 11, 2021 at 19:03
• @Tyberius I guess one should calculate the diabats, and analyze them for charge transfer character. Jun 12, 2021 at 3:28