Firefly and PC GAMESS-related discussion club

Learn how to ask questions correctly

Alex Granovsky
gran@classic.chem.msu.su

>Thanks for your attention. When the system is severely contaminated why the spins in x and y directions are changed and the Sz remains unchanged ?

This is because by its construction the approximate UHF wavefunction is eigenvector of S_z operator with
eigenvalue of (N_alpha-N_beta)/2, where N_alpha (N_beta) is the number of occupied alpha (beta) orbitals.

>In the case with s-squared = 5.1, can one report the result of the calculation as a proper local minimum with "3" multiplicity (let us suppose that there is not any imaginary frequency) or the spin was changed and the system seems to be a meta-stable ?

Generally, this means that UHF does not describe your system well enough. You may consider to perform calculations with MULT=5 or witch to MCSCF.

>The SCF convergence of ROHF method is worse than the UHF and sometimes the ROHF SCF procedure never converge.        

Yes, this may be the case. Your system does not like to be triplet :)

Kind regards,
Alex Granovsky

>Best Wishes
>Masoud
>--------------
>Masoud Nahali, Sharif University of Technology
>
>
>On Sat Feb 26 '11 1:19pm, Alex Granovsky wrote
>----------------------------------------------
>>Dear Masoud,

>>s-squared is the expectation value of S² operator:

>>S²=S_x²+S_y²+S_z²

>>>Spin Sz =1.0
>>>s-squared = 5.1
>>This means that this particular UHF wavefunction is severely
>>contaminated by higher spin multiplicities as for pure-spin
>>wavefunction one would get s-squared (high-spin case) = Sz*(Sz+1) = 1*(1+1) = 2
>>You can try to use ROHF as it always gives pure-spin answers.

>>Kind regards,
>>Alex Granovsky
>>
>>
>>
>>On Thu Feb 24 '11 11:55pm, Masoud Nahali wrote
>>----------------------------------------------
>>>Dear Firefly Users
>>>
>>>
>>>In ROHF and UHF calculations Firefly writes s-squared in the output file. If the meaning of the s-squared is
>>>the mean of squared spin value (s**2), please let me know that the "s" represents the total spin angular momentum or not ? or in other words I want to know that Firefly takes into account the spin in x and y directions or not ? Also for instance, I have run a job which its MUL is equal to 3 and in the output file I have found :

>>>Spin Sz =1.0
>>>s-squared = 5.1

>>>certainly the Sz=1.0 seems to be correct but the s-squared value is unclear for me ! I appreciate your help in advance to understand it.
>>>
>>>
>>> Best Wishes

>>> Masoud
>>>--------------
>>>Masoud Nahali, Sharif University of Technology        
>>>