Photochemical processes often involve dissociation of bonds in excited electronic states of molecules. We find that the workhorse of excited state quantum chemistry, time dependent density functional theory (TDDFT), is however fundamentally incapable of describing excited states for highly stretched single bonds, even if the underlying reference state is qualitatively correct. This is on account of the lowest triplet state spuriously disappearing at large separations, which indirectly also causes errors in predicting the character/energies of other states. Spin-flip TDDFT (SF-TDDFT) methods can correct the problem at the dissociation limit, but they do not predict continuous energy surfaces overall, rendering them unsuitable for studying the dynamics of photodissociation. Orbital optimized spin-flip methods or linear response on complete active space self-consistent field (CASSCF) with small active spaces were also found to be unviable, due to the presence of multiple local minima as solutions to the energy minimization problem. Two potential solutions to this problem are developed and discussed: one that is state specific and one that is capable of generating many excited states all at once. The former involves variance minimization of a mean-field Hamiltonian via adaptive gradient descent in orbital space, where each state only costs as much as a ground state DFT calculation, although strict orthogonality between states is slightly compromised. The more expensive, multi-state solution is to perform linear response on a perfect pairing (PP) reference wave function instead of the unrestricted DFT reference used in TDDFT. The resulting equation of motion PP (EOM-PP) approach should be able to describe the dissociation of isolated ensembles of single bonds even for excited states and predict smooth energy surfaces. The computational cost of EOM-PP scales the same as TDDFT (with a roughly 4 times larger prefactor), causing it to be applicable to systems where TDDFT is normally employed. Representative examples of both approaches will be described, along with corrections for improving quantitative performance.

Development of excited state quantum chemistry methods capable of describing photodissociation of single bonds

2019 Fall National Meeting

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