LAUSR

Abstract We propose a spin-tensor (ST) version of the multireference first-order polarization propagator (PP) approximation (MRFOPPA) designed to produce precision-balanced approximations (PBA) of electronic transition energies, Δ E , to low-lying electronic states of selected multiplicity in small molecules in a wide range of their nuclear geometry. Comparing the STMRFOPPA Δ E estimates with those obtained by the full configuration interaction (FCI) method and several multireference approximations on the example of transitions from the ground state of CH+ to its 25 excited states in the aug-cc-pVTZ basis set prompts the following conclusions: (i) STMRFOPPA provides PBA to the corresponding FCI results for the energies of transitions to the noted electronic states and the associated vibronic states and (ii) the accuracy of STMRFOPPA Δ E estimates is higher than that of MRCIS and close to those of MRCISD and XMCQDPT2 that operate in spaces of higher dimensions. Additionally, Supplementary Material to the paper contains the FCI(frozen core)/aug-cc-pVTZ level data for potential energy curves of the 26 electronic states of CH+ that can be useful for future research groups.