I am not a specialist in DFT and therefore do not wish to stick my head out. Here is a super useful weblink which can get you started:

http://www.huntresearchgroup.org.uk/teaching/year1_lab_start.html

From experience, even just unusual M-L distances can cause convergence issues. I know you probably aren't in a position to divulge your ligand structure, but check all M-L distances and angles. You can refer to a number of works from Prof. Karsten Mayer who, if memory serves me right, works with U chemistry. Where possible, construct your structure from coordinates obtained from XRD data.

I will keep following this question and hopefully there is an expert out there who can provide you with better answers.

Best wishes with the optimization!

P.S. Also check this out:

https://www.researchgate.net/post/Does_anyone_know_why_failure_is_occurring_in_optimization_with_6-31G?tpr_view=XTAqaa6nvLHNFcCd1HEcY764vS0Zdz1Y3kF1_1

In the computational papers dealing with U I've seen, the most commonly used basis set is the Stuttgart RSC 1997 ECP for U. Try this basis set, as well as LANL2DZ and CRENBL (you can get all three at https://bse.pnl.gov), and see if you can obtain converegce and reasonable geometries. Also, you can think of upgrading the light atoms basis set to 6-31G* (or 6-31G**).

There is also a set of gaussian keywords that help when convergence is difficult to obtain; try these:

opt=z-matrix or opt=cartesian

scf = qc or scf = xqc or scf

and different comobinations of them.

If that doesn't help, you can try changing the starting geometry and think about changing the DFT functional to any other modern one (PBE, M06).

Adding to Prof. Dr. Trzaskowski's response, I have used B3LYP / LANL2DZ and M06 / LANL2DZ and both have worked for me. Metals involved: W, Mo, Pt, Ti.  Other atoms: C, H, N, O, F, Cl. I am not too sure about what should be used for Uranium, but I would follow Prof. Dr. Trzaskowski's suggestions.

Mr. Lehman,

Try with Opt=gdiis and scf=qc, because of usually by this opt method there are found stationary points for complexes of f-elements. An eaxmple [UC]0 (below).

freq rb3pw91/sdd

----------------------------

! Initial Parameters !

! (Angstroms and Degrees) !

-------------------------- --------------------------

! Name Definition Value Derivative Info. !

--------------------------------------------------------------------------------

! R1 R(1,2) 1.523 estimate D2E/DX2 !

--------------------------------------------------------------------------------

Trust Radius=3.00D-01 FncErr=1.00D-07 GrdErr=1.00D-06

Number of steps in this run= 20 maximum allowed number of steps= 100.

GradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGrad

Input orientation:

---------------------------------------------------------------------

Center Atomic Atomic Coordinates (Angstroms)

Number Number Type X Y Z

---------------------------------------------------------------------

1 92 0 0.000000 0.000000 0.000000

2 6 0 0.000000 0.000000 1.522991

---------------------------------------------------------------------

...................................

----------------------------

! Optimized Parameters !

! (Angstroms and Degrees) !

-------------------------- --------------------------

! Name Definition Value Derivative Info. !

--------------------------------------------------------------------------------

! R1 R(1,2) 5.5491 -DE/DX = 0.0 !

--------------------------------------------------------------------------------

GradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGrad

...........................

Harmonic frequencies (cm**-1), IR intensities (KM/Mole), Raman scattering

activities (A**4/AMU), depolarization ratios for plane and unpolarized

incident light, reduced masses (AMU), force constants (mDyne/A),

and normal coordinates:

1

SG

Frequencies -- 231.4668

Red. masses -- 12.5730

Frc consts -- 0.3969

IR Inten -- 296.6381

Atom AN X Y Z

1 92 0.00 0.00 0.05

2 6 0.00 0.00 -1.00

-------------------

- Thermochemistry -

-------------------

Temperature 298.150 Kelvin. Pressure 1.00000 Atm.

Atom 1 has atomic number 92 and mass 238.05080

Atom 2 has atomic number 6 and mass 12.00000

Molecular mass: 250.05080 amu.

Principal axes and moments of inertia in atomic units:

1 2 3

Eigenvalues -- 0.000001256.241261256.24126

X 0.00000 0.00000 1.00000

Y 0.00000 1.00000 0.00000

Z 1.00000 0.00000 0.00000

This molecule is a prolate symmetric top.

Rotational symmetry number 1.

Warning -- assumption of classical behavior for rotation

may cause significant error

Rotational temperature (Kelvin) 0.06895

Rotational constant (GHZ): 1.436620

Zero-point vibrational energy 1384.5 (Joules/Mol)

0.33090 (Kcal/Mol)

Warning -- explicit consideration of 1 degrees of freedom as

vibrations may cause significant error

Vibrational temperatures: 333.03

(Kelvin)

Zero-point correction= 0.000527 (Hartree/Particle)

Thermal correction to Energy= 0.003401

Thermal correction to Enthalpy= 0.004345

Thermal correction to Gibbs Free Energy= -0.025561

Sum of electronic and zero-point Energies= -521.830718

Sum of electronic and thermal Energies= -521.827844

Sum of electronic and thermal Enthalpies= -521.826900

Sum of electronic and thermal Free Energies= -521.856805

E (Thermal) CV S

KCal/Mol Cal/Mol-Kelvin Cal/Mol-Kelvin

Total 2.134 6.761 62.942

Electronic 0.000 0.000 0.000

Translational 0.889 2.981 42.450

Rotational 0.592 1.987 18.624

Vibrational 0.653 1.793 1.868

Vibration 1 0.653 1.793 1.868

Q Log10(Q) Ln(Q)

Total Bot 0.571511D+12 11.757025 27.071550

Total V=0 0.999021D+12 11.999575 27.630042

Vib (Bot) 0.850367D+00 -0.070394 -0.162087

Vib (Bot) 1 0.850367D+00 -0.070394 -0.162087

Vib (V=0) 0.148647D+01 0.172156 0.396405

Vib (V=0) 1 0.148647D+01 0.172156 0.396405

Electronic 0.100000D+01 0.000000 0.000000

Translational 0.155417D+09 8.191498 18.861620

Rotational 0.432435D+04 3.635921 8.372017

***** Axes restored to original set *****

.....................

Hi Sean,

You've omitted some fairly key information. What is the oxidation state of the U centre in your complex? If it's anything but +6, you have unpaired 5f electrons which will give an electronic structure which will be difficult, but by no means impossible, to converge. Your chosen spin-multiplicity will also have an effect here (typically maximum multiplicity is most appropriate). the f-elements experience quite weak crystal fields (although these are often stronger for U than for e..g lanthanides and later actinides) and this means that for the open-shell case an appropriate single electronic configuration may be difficult to identify, and may even be qualitatively incorrect. Assuming that DFT can be applied reliably here, then the SDD ECP of Dolg et al which replaces 60 core electrons is definitely a good choice and also takes care of the scalar relativistic effects. Your basis set is small, but should be OK to get a converged electronic structure (even if not a particularly accurate one) if you're not dealing with an anionic system. You're using a hybrid functional which should help with increasing the energy gap between occupied and virtual orbitals, but there are probably several level-shifting options in Gaussian which you should investigate which will typically help in cases where convergence is difficult. Many quantum chemistry codes have a "Fermi smearing" option which is often useful in getting initial convergence of the electronic stucture and should help here, although you have to be careful using such an option and ensure that your final occupation numbers are integer.

Finally, as others have mentioned, ensure that you're starting molecular geometry is reasonable. Nothing will help very much if your starting geometry is very unrealistic.

Best wishes,

Andy

Hello Andy,

Thank you very much for the helpful information. The complex I am working on is actually neutral. The oxidation state is (IV) of the Uranium and I have assumed it is the triplet electronic state. I have cleaned up the starting geometry quite a bit and it seems to be tetrahedral around the U center which seems quite reasonable for my complex.

Hi Sean,

It might be that the tetrahedral symmetry is causing the problem. the 7 5f-orbitals split into 1 non-degenerate orbital and two sets of 3 degenerate orbitals in tetrahedral point groups. This means that there is no non-degenerate high-spin 5f2 configuration. In this case you would either need to lower the symmetry of your complex to break the degeneracy, or use a multiconfigurational approach. If you can perform some kind of orbital smearing in your Gaussian calculation this will identify if this is the problem, since if it is you will find that you have several f-orbitals with near-identical non-integer occupation numbers.   

Best wishes,

Andy

Hi Sean,

I agree to some extent to few of the comments given above. Uranium is too heavy and significantly affected by relativistic effects. We also see a bunch of theoretical papers (especially molecular property calculations) using completely non-relativistic methodologies. They claim that they took care of relativistic effects by using ECPs. In my opinion, that is not sufficient. The Hamiltonian (1 component, two component, four-component) has a considerable effect on the calculations for very heavy atoms. A protocol that gives reasonable results for first row metals will not give reasonable results for the heavy ones (the NRel methodologies predict silvery gold, and relativistic methodologies predict yellow, and yet U is too heavy than gold). In this case Professor Pekka Pyykkö's works are very important: (http://www.chem.helsinki.fi/~pyykko/pekka/pekka.html).

My advice: if you have access, go for SO-ZORA in ADF (two-component relativistic calculation in Amsterdam density functional). In ADF there are also basis sets optimized for relativistic calculations in addition to the possibility of using a two-component relativistic Hamiltonian. If not, always using relativistic basis sets is advisable. In case you need much better relativistic basis sets, have a look at the DIRAC website for Dyall's basis sets (if you need details,let me know).    

Good luck!

Taye