Near‐Hartree‐Fock assessment of reorganization effects in ionic states of acetylene

Calculations on acetylene 2Πu and 2Πg ions and the 1Σg neutral ground state using a series of wavefunctions with increasing basis set size converge to an estimated Hartree‐Fock vertical ionization energy near 10.0 eV and an estimated Hartree‐Fock vertical electron affinity near −4.2 eV. The calculations reveal that the near‐Hartree‐Fock σ orbitals change strongly with the state of ionization leading to σ‐electron energy errors (for transferring ground state σ orbitals to the ions) of − 4.5 eV for the cation and + 3.2 eV for the anion. Each carbon atom is calculated to lose only about 0.34 e upon π‐electron ionization and to gain about 0.42 e on π‐electron capture. Consequently there is poor transferability of the ground state HF σ core to the ionic states so that π‐electron energy changes of + 3.2 eV for the cation and − 3.8 eV for the anion are incurred. The error incurred by transferring π orbitals from the ground state to the ions with frozen σ core is −0.8 eV for the cation and − 0.35 eV for the anion...