Simple Models for Difficult Electronic Excitations

Abstract

We present a single-determinant approach to three challenging topics in the chemistry of excited states: double excitations, charge-transfer states, and conical intersections. The results are obtained by using the Initial Maximum Overlap Method (IMOM) which is a modified version of the Maximum Overlap Method (MOM). The new algorithm converges better than the original, especially for these difficult problems. By considering several case studies, we show that a single-determinant framework provides a simple and accurate alternative for modeling excited states in cases where other low-cost methods, such as CIS and TD-DFT, either perform poorly or fail completely.

Exotic Bonding Regimes Uncovered in Excited States

“…The two 2c,1e bonds in the T1 state of He 2 at its equilibrium distance.L eft, Schemes howing the two independentd elocalization channels. Evolution of the EDF in LiF.T he left population is that of the Li atom.S tructures are identified easily: p(2,10) for S0 and p (3,9) for S1 tend to 0a tl arge distance, while p (3,9)for S0 and p(2,10) for S1 tend to 1i nthe same limit. The contributions of each NAdO to the DI is also shown.…”

“…Figures 13 and 14 show the evolution of the CH 2 ÀCH 2 DI and EDF contributions. Interestingly,t he single-electron exchange (7,9) and (9,7) RSRSs are almostu naltered during the twisting phase. The correlated DI of the ground state is 1.63, at ypicalv alue foradouble bond.…”

“…There are only two RSRSs contributing non-negligibly to the EDF except at small distances:t he neutral (3,9) and the charget ransfer ionic (2,10) RSRSs.R emarkably,t he avoidedc rossingc an be neatly located from the contact pointo ft he neutrala nd ionic states at p = 1/2. The (2,10) component in one state is equal to the (3,9) component in the other and vice versa. Even more interesting is the fact that from the dissociation limit to R % 5 the S0 and S1 EDFs are almost exact mirror images.…”

“…Since only the (2,10) and (3,9) RSRSs are important, we may envision an electron flowing from the Li atom towardt he Fa tom in the S0 state as we approach the crossing from dissociation and an electron flowing from F À to Li + in the S1 state. Since only the (2,10) and (3,9) RSRSs are important, we may envision an electron flowing from the Li atom towardt he Fa tom in the S0 state as we approach the crossing from dissociation and an electron flowing from F À to Li + in the S1 state.…”

“…[2,3] Similarly, many biological processes involvel ight absorption and light harvesting. Recent methodological advances have gone a long way towards this end and, together with rigoroust echniques, such as the equationo fm otion (EOM) methoda nd its variants, [7] lower-cost procedures such as time-dependent DFT [8] and the maximum-overlap method (MOM), [9] have appeared. [6,4] Under such circumstances, the availability of accurate theoretical modelsa nd tools to study these states of matter becomes crucial.…”