What to do if Q-Chem can't determine orbital irrep?

In some rare cases, I run into an issue where Q-Chem could not determine the irrep of some orbitals in a symmetric system. This typically happens in the limit of stretched bonds or non-interacting dimers.

The warning I get is:

Warning : Irrep of orbital( 91) could not be determined

Any tips to get around this issue?

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This is a really annoying problem! I believe it happens because of finite numeric thresholds and most often we observe it when there are degenerate orbitals, like in linear molecules or in the non-interacting fragments that you described.
Because any linear combination of degenerate orbitals is valid, the SCF procedure can mix them, unless there are special controls. I also think it happens more often with large bases with diffuse orbitals.

There are several things to try:

  1. Use new integral library by setting ‘use_libqints=true’, it shows a more robust behavior.
  2. Do a fake DFT calculation first (using the grid helps to resolve symmetry
    of degenerate irreps):

    exchange gen

    X HF 1.0
    X Slater 0.000001

and then read the orbitals into the second job using ‘scf_guess = read’

  1. Sometimes playing with guess and SCF algorithms can help.

  2. If desperate, try to use a smaller basis.