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Re^3: Free-radical optimization in Mp2

sanya
sanya@photonics.ru

Of course, you cannot.

To perform vibrational analysis, you should have a geometry optimized at the level you're going to use for the vibrational analysis. During geometry optimization, you obtain a stationary point (i.e., the structure with gradients less than opttol parameter). It may be a minimum, but may be not. Obviously, the structure optimized at a different level is not a stationary point for the level chosen. So. you'll get a message: "This is not a stationary point, the vibrational analysis is not valid". The resulting thermodynamical data will be meaningless.

If you are studying the molecule from this post: http://classic.chem.msu.su/cgi-bin/ceilidh.exe/gran/gamess/forum/?C331915db42mN-8079-958+00.htm, it is NOT a biradical. To make it a biradical forcibly, you should use either GVB or CASSCF. Otherwise, it will be close-shell whatever you do. ROHF is only for high-spin open shells, while you case (homolytic bond cleavage) is a low-spin open shell.

The problem is that for homolytic bond cleavage, restricted HF methods are inadequate and give wrong dissociation energy and the shape of the dissociation potential curve as a whole. At near-equilibrium distances, RHF solution is OK; it is a good approximation to the true wavefunction. On the contrary, at large internuclear separation, the true wavefunction is better approximated by an open-shell solution (at infinite separation, open-shell triplet and singlet states are degenerate).

On Fri Mar 2 '12 5:22am, gensz wrote
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>And, can I do the energy calculation in MP2/6-311++G** and then the Vibrational Analysis at ROHF/6-31G**? then the Electronic energy from MP2/6-311++G** + ZPE + Translational + vibrational + rotational energies from HF/6-31G**? is it okay to add them up even I calculated the vibrational analysis into  different Basis Sets and LEvel of Theories?..
>Thanks

[ This message was edited on Fri Mar 2 '12 at 10:05pm by the author ]

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