Calculate Heat of formation by Guassian98

From: Hu LiHong <lhhu &$at$& yangtze.hku.hk>
Subject: Calculate Heat of formation by Guassian98
Date: Thu, 2 Jan 2003 13:09:43 +0800 (HKT)

 Hello all,
 Happy new year!
 I'd like to use Guassian to calculate the heat of formation of some
 compounds. I am using DFT method and basis set 6-311G(d,p) to optimize
 the structures and calculate the frequency. After checking the website of
 www.gaussion.com, I used the formula for calculating the enthalpy of
 formation, it is a little complicated but I found the result of enthalpy
 of formation have large difference from experimental values. I don't know
 what's wrong with it?
 Who can tell me 1) whether I can use the formula in the website
                 2) what should be noticed when I am using it
                 3) if there is other easier method to calculate the
                    enthalpy of formation by using the energy calculated by
                    Guassian 98?
 I'll appreciate you very much
 Cheer,
 Yours,
 Carol