Charge Migration in Cations: Structures, Energies, and Vibrational Frequencies of Ground and Excited States of HCCX+, CH3X+, Vinyl Bromide Cation and Propiolic Acid Cations

Abstract Coherent superposition of ground and excited states of cations can produce ultrafast charge migration that can be monitored by attosecond transient absorption, attosecond pump-IR probe, high harmonic spectroscopy, attosecond streaking, and strong field ionization. Vibrational motion on the ground and excited states of the cations leads to decoherence and decay of charge migration. Computational modeling of charge migration requires suitable methods for calculating energy differences between the electronic states of the cations and potential energy surfaces for the ground and excited states of the cations. The performance of CCSD, EOM-CCSD, EOMCC-IP, CASSCF, and MC-PDFT/L-PDFT has been examined for computing ionization energies, cation excitation energies, and harmonic vibrational frequencies for ground- and excited-state cations. Three sets of molecules that are relevant to charge migration were selected to illustrate different aspects of the calculations: HCCI+ and HCCBr+ have degenerate ground and first excited state of the same symmetry; CH3I+ and CH3Br+ are subject to Jahn–Teller distortion in their ground state; charge migration in vinyl bromide cation and propiolic acid cation involves the ground state and second excited state of the cations. CCSD is limited to the lowest energy state of a given symmetry. EOM-CCSD can be used for excited states. Both are easy to apply, “black box” methods. EOMCC-IP is an alternative that is less expensive than EOM-CCSD and is suitable for both ground and excited states of cations with degenerate vibrational modes. CASSCF with state averaging is needed for Jahn–Teller distortion cases. MC-PDFT is suitable for adding correlation energy to CASSCF and generally improves the CASSCF results. L-PDFT is required for the proper treatment of cations with degenerate or closely spaced electronic states.