Subject: MP2/UMP2 vs B3LYP/UB3LYP

From: Valentine Ananikov <[email-masked]>
Date: Wed, 17 May 2000 09:29:22 +0400

Dear CCL,

I am a little surprised with the following problem and
would be grateful for helpful comments.

On a substituted carbene compound MP2/UMP2 optimization of
singlet and triplet states respectively leads to energy gap
of about 9 kcl/mol and low lying singlet state. However,
in the case of B3LYP/UB3LYP optimized structures triplet
state is more stable by approximately the same value.
Geometry parameters are also different. Unfortunately,
experimental data for this compound is not available.

Is this a known thing ? any precedents ?

Does anybody have an idea on the possible explanation and what
accuracy one would expect from MP2 and B3LYP calculated s/t gaps?

Thank you!

best regards,  Valentin.

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