Partition Scheme Using A Function

I can't seem to find a way to do the following:

create table part_table (
col1 int,
col2 datetime
) on psX (datename(week,col2))

I want to partition based on the week number of a date field.

So if I enter in data like the following in my part_table:

(1, 1/1/2007) should go into partition 1 for week #1
(52, 12/21/2007) should go into partition 52 for week #52 of the year

I tried adding in a computed column, but it says its nondeterministic.