Is there anyway to create a functional index using to_date?

I'm running 7.3.4 and we are having problems with our reporting views
on our database.  One fix I wanted to do was to create a functional
index using the to_date function.  However, when I do a create index
abc on table def(to_date(char_field,'yyyymmdd')), PostgreSQL complains
about a parser error at the 'yyyymmdd'.  Is there anyway to force
PostgreSQL to create this kind of index?

I can get it to create on if I create a special function that just
takes the field to be converted and returns the converted date.
However, I would prefer not to go that route since it requires us to
update all of our views.

Thanks for any help,

Chris