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Re^3: How to go about the ODD number of electron configuration?

Alex Granovsky

Dear Siddheshwar,

RHF means Restricted Hartree-Fock method and can only be applied
to closed shell systems i.e. to systems where each spatial orbital
has occupation number equal either to 2 or to 0. More precisely,
there is exactly one alpha and one beta electron populating
each occupied orbital. In addition, all degenerate shells
(i.e. orbitals of equal energy belonging to the degenerate
irreducible representation of the molecular point group) should
be either all non-populated at all or completely populated.
For instance, singlet O2 molecule in its ground state
is not closed shell system as there are only two electrons on
two degenerated pi orbitals.

While ROHF formally means Restricted Open-Shell Hartree-Fock
method, it is usually assumes a high-spin coupling of unpaired
electrons. For instance, with Firefly ROHF is used to designate
Restricted High-Spin Open-Shell Hartree-Fock method (note, other
spin coupling situations can be handled by GVB code).

High spin means that the spins of all unpaired electrons are the
same (i.e. either all alpha or all beta) As this two situations
are equivalent, ROHF code in Firefly assumes that unpaired electrons
are all of alpha spin. In addition to high-spin coupling scheme,
ROHF also assumes that all degenerate shells are either non-populated
or completely populated, or are exactly half-populated (i.e. there is
exactly one alpha and no beta electron per each component of degenerated shell).

Exactly as RHF, ROHF implies that a pair of alpha and beta
spin-orbitals is constructed using the same spatial function
called orbital, and this is what the word Restricted means.
If one allows alpha and beta spin-orbitals to have independent
spatial orbitals, one arrives at UHF or Unrestrcted Hartree-Fock

Both ROHF and UHF can be used to model systems with unpaired
electrons. Each of them has specific advantages and disadvantages.

For further information and more details on these methods,  
I would recommend you to read a good book on quantum chemistry.

Kind regards,
Alex Granovsky

On Mon May 6 '13 6:08pm, Siddheshwar Chopra wrote
>Dear Farid,
>Yes the question was simple. Just that I havent come across such situation before.. Could u tell the difference between RHF and ROHF?? Or where do I use ROHF???

>On Sun May 5 '13 7:54am, Farid wrote
>>Is this a trick question?  Don't you just change the multiplicity?  Maybe under $contrl use mult=2?  Or am I not understanding your question?

>>On Thu May 2 '13 3:13pm, Siddheshwar Chopra wrote
>>>Dear Sir,
>>>I faintly understand what ROHF is... And I get error whenever I run RHF calculation for an ODD no. of electron configuration.. I guess I can add one Hydrogen somewhere and do the calculations with RHF.. But WHAT if I need to study the structure with ODD no. of Hydrogens (or electrons).. Please throw some light on this. Even if I add ICHARG, then also my purpose is defeated. .Am I correct?


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