7.9 Coupled-Cluster Excited-State and Open-Shell Methods

7.9.8 Charge Stabilization for EOM-DIP and Other Methods

The performance of EOM-DIP deteriorates when the reference state is unstable with respect to electron-detachment,Kus:2011, Kus:2012 which is usually the case for dianion references employed to describe neutral diradicals by EOM-DIP. Similar problems are encountered by all excited-state methods when dealing with excited states lying above ionization or electron-detachment thresholds.

To remedy this problem, one can employ charge stabilization methods, as described in Refs. Kus:2011, Kus:2012. In this approach (which can also be used with any other electronic structure method implemented in Q-Chem), an additional Coulomb potential is introduced to stabilize unstable wave functions. The following keywords invoke stabilization potentials: SCALE_NUCLEAR_CHARGE and ADD_CHARGED_CAGE. In the former case, the potential is generated by increasing nuclear charges by a specified amount. In the latter, the potential is generated by a cage built out of point charges comprising the molecule. There are two cages available: dodecahedral and spherical. The shape, radius, number of points, and the total charge of the cage are set by the user.

Note:  1. A perturbative correction estimating the effect of the external Coulomb potential on EOM energy will be computed when target state densities are calculated, e.g., when CC_EOM_PROP = TRUE. 2. Charge stabilization techniques can be used with other methods such as EOM-EE, CIS, and TDDFT to improve the description of resonances. It can also be employed to describe meta-stable ground states.