I'm trying to optimize the geometry of a metal-organic framework (MOF) using ORCA 6.0.1. My system has about 110 atoms, and the central metal ion is Ni(II).

Method details:

Functional: B3LYP with D3BJ dispersion correction

Basis set: def2-SVP

SCF settings: TightSCF

Approximations: RIJCOSX

Other: default geometry optimization settings

However, the optimization does not converge, even after 300 optimization steps. The geometry fluctuates, and I don’t see meaningful progress in the energy or forces toward convergence.

Things I’ve tried:

Increased MaxIter to 300

Used TightSCF to stabilize SCF convergence

Started from a crystal structure

Tried cleaning up initial geometry in Avogadro

My questions ?

What changes would you recommend to improve convergence in this kind of large system (especially with a transition metal)?

Would switching to a smaller basis set like def2-SV(P) or using VeryTightSCF help?

Should I consider using Calc_Hess true, or applying constraints to linker atoms?

Any suggestions on strategies or input examples for large MOF optimizations in ORCA would be greatly appreciated!

Thanks in advance