Please help On Alternative Database Location

it's sure that this email should to be write to
anywhere but here, but the various requests to help
done in some mailings lists about postgresqk are
failed.So i write you, in the hope to receive an
response.
Briefly the question.
I work under Linux platform, Mandrake 7.2
distribution, and i need to create a postgres
database, for example called "mydb", in alternative
location, for example "/home/username". In the
official documentation this is very simple to do, but
in the reality i have failed.
This is my steps
1) su - username
2) set PGDATA2 = "/home/username" (but i tried
relatives path too)
3) initlocation $PGDATA2
4) createdb -D $PGDATA2 mydb

Finally an error message appears like this
"The database path '/home/username' is invalid. This
may be due to a character that is not allowed or
because the chosen path isn't permitted for databases"

I tried the relatives paths "~" and "." too, but the
message is the same.
Please help me
I like to know the right procedure step by step
Thanck you so much

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