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

7.8.7 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,507, 508 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. 507, 508. 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.