Subject: Non-adiabatic MD: multiple configurations?

From: [email-masked]
Date: Fri, 23 Apr 1999 15:10:37 +0100 (GMT)

I am attempting to repeat the calculations of Gersonde and Gabriel (J.
Chem. Phys. vol 98 (3), p 2094 (1993)) in which the photodissociation of
Cl2 in Xe is discussed.

I am attempting to describe the electronic degrees of freedom using a
simple one particle model (tight binding), but have found a problem which
I fear is fundamental. Since my background is in DFT, I am not well versed
in multiple configuration interactions, hence the following question.

Excited states within the model are created by promoting an electron from
a low lying level to an empty higher lying level. When the interactions
are weak I believe this is a reasonable description. But when they are
weak, I suspect the model breaks down.

Consider the Cl2 dimer. It is unstable with respect to dissociation when
placed into an excited state. Thus, on being excited by a photon the atoms
begin to separate. As this happens, the HOMO-LUMO splitting decreases
until they become nearly degenerate. Thus it is no longer possible to
distinguish the excited from the unexcited states. No transition between
states is observed to take place. This is a problem should the two atoms
meet up again some time later as it is not clear what state they should go
into - ground state or excited state.

I suspect that the root of the problem is that when the HOMO-LUMO
splitting is very small it no longer makes sense to work with a single
particle picture - it takes no energy to move between configurations, thus
they will mix freely.

At this point I get lost. Does this analysis really solve the problem of
what state the system is in? Or would there still be ambiguity even if
configurations were allowed to mix?

Cheers,

Andrew

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