Here, Ĥel (r, R(t)) is the electronic Hamiltonian operator for a set of nu-clear coordinates, R, at time t. Using an electronic structure method such as DFT, the electronic Hamiltonian can be solved to obtain the adiabatic eigenvectors Φ i (r, R(t)) and eigenvalues i (R(t)). These adiabatic eigenvectors can correspond to molecular orbitals, Slater determinants, etc., depending on the employed electronic structure method. Accordingly, the corresponding eigenvalues can represent the molecular orbital energies, ground or excited state energies, etc. By assuming that these adiabatic eigenvectors form a complete orthonormal basis, we can express the total wavefunction as their linear combination, i.e., |Ψ(r, t)