Dear colleagues, I would like to ask you for help.
I have been trying to calculate the energy of a binuclear Cr(II) organometallic complex, where the Cr(II) atoms are connected via hydride bridges, to determine the spin state and exchange coupling constant.
In these complexes, Cr(II) can exist either in a "high-spin" state with 4 unpaired d-electrons (S = 2), or in a "low-spin" state with 2 paired and 2 unpaired electrons (S = 1). I have calculated the energies of the former for ferromagnetic (multiplicity M = 9) and antiferromagnetic (M = 1) coupling of Cr spins, even the ferromagnetic coupling of the latter (M = 5). However, every time I try running a calculation for the antiferromagnetic coupling of the Cr(II) with spins S = 1, the calculation converges to the antiferromagnetic high-spin solution with Cr spins S = 2.
Please, do you have any idea how to persuade Gaussian to stick to the "low-spin" solution?
I use unrestricted broken-symmetry DFT in Gaussian 16 and basically follow the instructions in https://gaussian.com/afc/ - start from experimental geometry (or optimized from the ferromagnetic case), divide the molecule into 4 fragments (2x(Cr+ligands), 2x(H- bridge)), create a guess, and use the created checkpoint in follow-up optimizations. The first rows of the guess look like this (leaving out the basis definitions for C, H, and Cr at the end):
%chk=complex-a.chk
%nprocshared=4
%mem=4GB
# ub3lyp/gen pseudo=cards guess=(fragment=4,only)
Unrestricted 18134o
0 1 1 3 1 -3 -1 1 -1 1
C(Fragment=2) 16.75200000 9.35300000 2.77500000
...
The follow-up optimization, which then converges to the "unwanted" high-spin solution:
%chk=complex-a.chk
%nproc=4
%mem=8GB
#ub3lyp/gen opt freq scf=xqc guess=read geom=allcheck pseudo=cards
...
Already applying the stable=opt calculation on the generated guess provides the high-spin AFM solution. The above-mentioned input lines worked well with the ferromagnetic "low-spin" case, I don't understand why changing just a sign of the multiplicity of one fragment changes the output so dramatically...
Moreover, we know from magnetic measurements that this complex is antiferromagnetic and the XRD structure is closer to the "low-spin" FM geometry, so the solution I am not able to get with the calculations sketched above might be indeed the right one.
Please, I would be really grateful if anyone has any idea how to solve this trouble...
Hi. I did not get at all your explanation. I have a few question about your the fragmentation. Why do you use 4 fragments? Usually, it works just with two fragments. In the case of the 4 fragments, please BE AWARE of the charges and multiplicities of each one, the final state should be the Broken-symmetry state. The first optimization (or single point) must contain the wave function (WF) of the highest multiplicity. This WF of the HS system must be use to obtain the broken-symmetry solution for the low spin.
On the other hand, when you say: "why changing just a sign of the multiplicity..." What do you mean exactly? The multiplicity must be a positive integer number.
Finally, I suggest to use ORCA package to do this kind of calculations.
Hello Nelson David Arias Olivares, thank you for your message and your recommendation, I might try the ORCA package for future calculations.
My question is probably Gaussian-specific. In my calculations of the AFM complex with two Cr(II) centres, I fully proceed from both the Gaussian tutorial on AFM coupling and the book Exploring Chemistry with Electronic Structure Methods, where using 4 fragments (two additional ones for the mu-bridges between metal centres) is advised. The charges and multiplicities of individual fragments should be OK, otherwise, all the other calculations that I mentioned above would have failed. In Gaussian, one changes the orientation of the spin (alpha to beta) by changing the sign in the definition of multiplicity. I have already done all your suggestions when calculating the multiplicities M=9 and broken-symmetry M=1 for the two chromium(II) ions with 4 unpaired electrons (with success), even M=5 for Cr(II) ions with two unpaired electrons.
All I need now is the broken-symmetry M=1 solution for chromium(II) ions with two unpaired d-electrons...because my calculation converges to the broken-symmetry M=1 solution for Cr(II) ions with four unpaired d-electrons, even though the definition of multiplicities of individual fragments should be fine (M = 3 for each fragment containing Cr(II)).
I am sorry, I think that our misunderstanding stems from the term "HS" and "LS" - in my question, I relate it to the spin state of the individual Cr(II) ions, not the whole complex :-)
If you have any other idea, I would be happy to try it...
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