Dear all
I have 2 molecules and want to adsorb them one by one on Fe2O3 catalyst and then make products one by one in Gaussian 09.
1) I want to ask that during minimum optimization of Fe2O3 only, do we need to specify specific spin and the multiplicities in that spin or as by default whatever Gaussian takes? I have simulated for default spin using each multiplicities and looked at which multiplicity is giving minimum. Is that right?
2) Now when I got to know the ground state conformer and want to adsorb my molecules one by one then which multiplicity do I need to take care of in TS optimization? ground state mult. for whole complex (catalyst+substrate) or gr. st. mult. for catalyst and singlet for the substrate?
Please help me out. I'm tyro in Gaussian 09.......
Hi,
to answer your first question, do you use a cluster or a slab to model Fe2O3? In the Gaussian input, you should specify charge and spin for a system you would like to calculate, for instance, it could look like:
....
Job title
0 1
xyz coordinates of the system
where 0 is for charge and 1 stands for spin ("The first line of the molecule specification section specifies the net electric charge (a signed integer) and the spin multiplicity (usually a positive integer). Thus, for a neutral molecule in a singlet state, the entry 0 1 is appropriate. For a radical anion, -1 2 would be used. " http://gaussian.com/g_tech/g_ur/m_molspec.htm). Yes, you should check different spin multiplicities for the same charge. You cannot tell about "spin" and "multiplicity" separately.
To answer your second question, you should take care about the spin multiplicity for the whole system. Also, for more about optimization, please refer to this section of the Gaussian manual: http://gaussian.com/g_tech/g_ur/k_opt.htm
Hope this helps a bit.
Best regards,
Aleksey.
Hi Dr. Aleksey
Thanks for responding......
I have a single Fe2O3, not cluster.
Charge I have taken as 0 and spin as default (it automatically takes as restricted or unrestricted (Here I have confusion that do I need to take care of first, restricted and each multiplicities under this and then unrestricted and each mult. under this and find out which combination is giving ground state. Or just by default spin and each mult)).
Under this conditions (charge 0 and default spin) I have found minima for each multiplicities (1, 3, 5, 7, 9,....., 19) and nonet multiplicity has come as the minimum ground state condition. Now when I adsorb my substrate on this Fe2O3, do I need to keep nonet only for complex or again to find out the minimum before I proceed for transition state?
I think I messed up a little bit. If you don't understand then please let me know. I'll send you the images.
Dear Verma
You must select unrestricted method and nonet multiplicity for Fe2O3. But in TS you have new structure with new electron distribution thus u should find proper multiplicity for this state.
Sincerely
Ali
Dear Dr. Velma,
regarding your substrate, is it a singlet molecule? If yes, then, as Dr. Peyghan wrote, you should use the nonet multiplicity (the lowest in energy) for your Fe2O3 molecule and find proper multiplicity for the TS. By the way, in the Gaussian software, for example, you do not need to specify explicitly the unrestricted DFT method if you use it in your calculations: the program would use it by default if you calculated the system with other multiplicity than singlet.
Best regards,
Aleksey.
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