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Re^6: MCSCF for a large system.


Dear Sanya,
I got your insights: despite MCSCF is a technique for dealing with states and orbitals *simultaneously*, on average, the orbital perturbation part is dominating and in practice itís better not to touch states or at least keep them Ďpureí if you badly need to. Unfortunately itís not gona work for my problem. My protein activates triplet O2 molecule to do an oxidation job. O2 is too small to grab it and break apart. So the protein uses a cofactor that donates an (beta) electron from itís carbon-carbon double bond into alpha occupied orbitals of O atom. The donation is through coordinated by Fe2+ ion (cofactor and O2 doesnít ever meet in the active site). And reagents geometry is toughly restrained by surrounding inert protein body. Should one expect significant orbital perturbation due to transfer of beta electron only in a system of molecules with constrained geometry? I donít think so. What would change thus (as the reaction proceeds)? The configurations rate I believe. Does it make sense now?
The experimental part of my project is screening of cofactor candidates to speedup the reaction. Thus orbital things go into play much later, after cofactor initiated the oxygen (surely I see the shifts as breaking O-O bond, but they are not the knowledge I need to predict cofactor efficiency).

On Sun Sep 28 '14 3:50am, sanya wrote
>Did you mean wstate rather than nstate? as far as I understand, the problem is voluntarism in state-averaging, which is defined by wstate. As for nstate, you just can order more states than you're actually going to analyze, if your computer allows. Wstate is a bit more complex.

>First of all, it has nothing to do with thermodynamics (that it, it is _not_ the array of Boltzmann weights of the states). Wstate is just the array of weights used for construction the density matrix for orbital optimization. If you are interested in more than one state, you should obtain the orbitals equally good (or equally bad) for all the states of interest. If you give preference to one of the states, your resulting orbitals will be biased and, therefore, sort of meaningless.

>If you perform a state-averaged CASSCF for different geometries (say, along the reaction path), you'll see that the dominant configurations in your states of interest (say, ground and 1st excited state) change. These changes in the weights of individual configurations within the target states reproduce the changes in the corresponding wavefunctions along the reaction path. For example, the dominant configuration in the ground state was ...2200... (the digits are the occupation numbers) with 99% weight. As you move toward the transition state, the weight of this configuration will decrease, while the weight of ...2110... will increase. The state with dominant ...2200... configuration will become excited (therefore, state crossing should occur in some point). In the product state, you'll again have the ground state dominated by ...2200..., but the orbitals will be different, namely, product-like (rather than reagent-like). If you perform such a calculation, you'll see it clearly.

>So, the amount of states to be averaged is governed by the amount of terms that cross along your reaction path. If you're lucky, you'll need only closed-shell singlet, 1st excited singlet, and a triplet. This is typical, for example, for rotation around conjugated bonds in linear conjugated systems. Since I know nothing about you system, I cannot say anything about wstate a priori.

>Moreover, you may try state-specific CASSCF (that is, with default wstate(1)=1,-0). If you see that the dominant configuration of your ground state changes substantially along the reaction path, state-averaging is needed.

>As for the active space size... it does not have to be _large_, but is _must_ be balanced. For example, if you see symmetry breaking in the electron density where the nuclear configuration is symmetric, something is definitely wrong. In this case, you should carefully examine and change you state-averaging scheme, then revise your active space. The natural orbitals with occupancies >1.9< and 0.01 can be considered as inactive for the given state-averaging scheme and, therefore, can be excluded from the active space or replaced by some others.

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