The quantum chemistry community has developed analytic forces for approximate
electronic excited states to enable walking on excited state potential energy
surfaces (PES). One can thereby computationally characterize excited state
minima and saddle points. Always implicit in using this machinery is the fact
that an excited state PES only exists within the realm of the Born-Oppenheimer
approximation, where the nuclear and electronic degrees of freedom separate.
This work demonstrates through ab initio calculations and simple nonadiabatic
dynamics that some excited state minimum structures are fantastical: they
appear to exist as stable configurations only as a consequence of the PES
construct, rather than being physically observable. One such case is the S2
excited state of phosphine and a second case are local minima of a number of
states of tris(bipyridine)ruthenium(II). Each fantastical structure exhibits an
unphysically high predicted harmonic frequency and associated force constant.
This fact can serve as a valuable diagnostic of when an optimized excited state
structure is non-observable. Their origin lies in the coupling between
different electronic states, and the resulting avoided crossings. The upper
state may exhibit a minimum very close to the crossing, where the force
constant relates to the strength of the electronic coupling rather than to any
characteristic excited state vibration. Nonadiabatic dynamics results using a
Landau-Zener model illustrate that fantastical excited state structures have
extremely short lifetimes on the order of a few femtoseconds. Their appearance
in a calculation signals the presence of a nearby avoided crossing or conical
intersection through which the system will rapidly cross to a lower surface.Comment: 7 pages, 4 figure