I am working on the optimization of a transition state (TS) involving a cytochrome P450 model, which includes a porphyrin ring with an Fe center, a cysteine amino acid ligand, and a substrate. The full system consists of 132 atoms.
Would using the SCF=(XQC, NoVarAcc, NoIncFock) keyword in Gaussian be beneficial for improving convergence during this transition state optimization?
My recommendation would be to do ONIOM, with the smaller organic stuff at PM7 or something, and the metal and sp2 porphyrin ring with DFT.
For your extensive cytochrome P450 transition state comprising 132 atoms, it is advisable to employ SCF=(XQC,Tight) to ensure the convergence of the challenging electronic structure. This approach is significantly advantageous.
The paramount step involves computing a precise initial Hessian: incorporate Opt=(TS, CalcFC) within your optimization parameters. In the absence of this, the optimization process is prone to failure.
Refrain from using NoVarAcc and NoIncFock unless you observe distinct indicators of SCF failure, such as erratic oscillations.
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