The direct determination of the ionization potentials of CH and CH3 using the electron propagator method with a multiconfigurational second-order perturbation theory wavefunction as the initial state (EPCASPT2)

We directly determine the first two vertical ionization potentials (IPs) of two small organic radicals, CH and CH3, using a new method, the electron propagator method with a multiconfigurational second-order perturbation theory wavefunction (EPCASPT2) and using the multiconfigurational spin tensor electron propagator method (MCSTEP). Our IPs using EPCASPT2 are in excellent agreement compared to full configuration interaction (ΔFCI). For CH MCSTEP and EPCASPT2 give very accurate IPs to the lowest two cation states. For CH3, the lowest MCSTEP IP is inaccurate while the EPCASPT2 IPs effectively mimics ΔFCI IPs for all the IPs determined.