Self-Consistent Field Solution for Anions: Effect of the Removal of Pseudo-Continuum States

<title>Abstract</title> <p>This article introduces an innovative solution to prevent variational collapse for metastable anions by removing the sea of non-physical pseudo-continuum states at the SCF stage and implementing it in a non-Hermitian domain, where the lifetime of the electron attached state can be accessed. This SCF solution is achieved in two steps. First, a SCF calculation is performed on the neutral molecule, and the \textcolor{black}{valence antibonding orbital} is identified. Next, the extra electron is added to the \textcolor{black}{valence antibonding orbital}, and the SCF calculation for anionic species is carried out by modifying the Fock-matrix after removing the pseudo-continuum states present between HOMO and \textcolor{black}{valence antibonding orbital}. The resulting SCF solution corresponds to the metastable anion without any variational collapse. Here, Complex absorbing potential (CAP) is implemented at the Hartree-Fock (HF) level, and the solution obtained at the HF level is then used to perform the post-SCF calculation. Identifying pseudo-continuum states to be excluded for the post-SCF calculation becomes straightforward after the \textcolor{black}{valence antibonding orbital} is identified. Removal of these pseudo-continuum states does not have a significant effect on the correlation energy. This SCF solution is highly advantageous, as it contains a reduced number of MOs for post-SCF calculations, thereby saving significant computational time.</p>