Self-consistent-field solution for unstable anions

For an unstable anion, when using a diffuse basis set, the self-consistent field (SCF) solution often leads to a minimum energy solution with an additional electron in the most diffuse orbital present in the basis set. Eventually, it converges to a neutral molecule and a free electron with increase in the diffuse functions. Consequently, any post-SCF calculations using this solution do not yield meaningful physical results. This paper introduces the use of modified parametric equation of motion (PEM) in combination with nuclear charge stabilization to obtain a desirable SCF solution for an unstable anion. The SCF solution for an anion at a higher nuclear charge is used as the starting point, where the anion is bound. The modified PEM is then carried out until the additional nuclear charge reaches zero, ultimately producing the SCF solution for the unstable anion. The key to achieving meaningful SCF solutions for temporary anions lies in maintaining a fixed occupation number for the orbital corresponding to the singly occupied molecular orbital throughout the PEM calculation. The solution obtained using the modified PEM is checked for being self-consistent-field solution, confirming the accuracy of the method. To account for electron correlation, MP2 calculations are carried out for anionic species.