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I am running (opt+freq)DFT calculations using 6-311G(d) basis set on polymers with different number of monomer units. Upto 2 monomer units optimization is converged but for 4 units max displacement criteria is not converged. Which starting point do I need to consider while increasing the basis set?

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    $\begingroup$ What do you mean by increasing the basis set? Do you mean using a larger basis set? For the 4 unit polymer does the optimization run out of steps? $\endgroup$
    – S R Maiti
    Commented Aug 6, 2022 at 9:42
  • $\begingroup$ Sometimes for large molecules, it can take a long time for optimizations to converge because the torsions are low-energy and take many steps to settle. $\endgroup$ Commented Aug 6, 2022 at 19:07

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