HIGH LEVEL AB INITIO INVESTIGATION OF INTERCOMBINATION BANDS OF C$_2$

Abstract

High level \textit{ab initio} calculations were carried out on the $X^1\Sigma^+_g$, $A^1\Pi_u$, $a^3\Pi_u$, $b^3\Sigma^-_g$, $c^3\Sigma^+_u$ and $d^3\Pi_g$ levels of the C$_2$ radical. Calculations were carried out at the MRCI level with full-valence CASSCF reference wavefunctions and basis sets up to aug-cc-pv6z. The resulting wavefunctions and potential energy surfaces were employed to calculate Einstein coefficients for allowed transitions and intercombination bands. These values were used to model the behaviour of the C$_2$ system in interstellar and circumstellar environments, paying particular attention to the Red Rectangle (HD 44179).