Subject: force constants of diatomics

From: Dmitry Rozmanov <dima)at(xenon.spb.ru>
Date: Wed, 14 Jul 2004 17:09:15 +0400

Hello CCL.

I have calculated some diatomics in Gaussian03 for self educational purpose and 
stuck with a problem.

I got some meaningful constans for HF, HCl, HBr (10.66, 5.22, 4.19, 3.23 
mDyne/A). I found somewhere these constants as 9.70, 4.80, 4.10 and 3.20. Then 
there is no problem here.

However, for CO and NO I got something strange, 41.34 and 39.29. While I have 
found 18.60 and 15.30 mDyne/A for these species. This is the problem. Why are 
they two times greater?

Have anybody seen such a behaviour before?

The calculations were RHF/6-31G (ROHF for NO and LanL2DZ for HI).
I have tried MP2/6-31++G(d,p) for CO and got 35.18 force constant. The same 
thing anyway.

I was told that G98 produces the same results. By unknown reason.

What do I do wrong?
How to overcome the problem?

Thank you in advance.
Best wishes.

	---Dmitry.