Thermodynamic analysis for weakly bound complexes



I am trying to understand why I can’t get rid of imaginary frequencies for the complexes I am analyzing. The problem exists for both strongly bound (water-water) and weakly bound (nitrogen-nitrogen) complexes.

  • I tried optimizing the geometries using a tighter convergence criteria. Optimized geometries resulting from INTERFRAG_TIGHT, GAU_TIGHT, and GAU_VERYTIGHT were passed to frequencies and there are still imaginary frequencies.
  • I also tried using analytic gradients and played around with a tighter disp_size and still get imaginary frequencies.
  • I even started with optimized geometries from the S66 and A24 databases and am still getting imaginary frequencies.

I attached a couple of example input and output files for the water-water complex.

water_water_A24_summary.txt (1.1 KB)
water_water_A24_out.txt (569.5 KB)
water_water_A24_in.txt (2.6 KB)

water_water_summary.txt (1.2 KB)
water_water_out.txt (692.8 KB)
water_water_in.txt (2.8 KB)

Are imaginary frequencies just a part of frequency analysis on weakly bound complexes? Or is there something I can do to eliminate them.

Thanks in advance for any help you can provide!



Weakly bound complexes don’t have imaginary modes at stationary points, but their normal modes and frequencies are sensitive to having “things” exactly right.

In your case, this looks like something not being exactly right in your input file. You optimize at DF-MP2, then freeze the core, and then take frequencies at fc-DF-MP2. DF-MP2 is not fc-DF-MP2. When I freeze the core, then optimize, then take frequencies, I get all real modes, as expected.

Let me know if this doesn’t solve your problem.


I went ahead and marked my response as solving the problem. Let us know if you’re still having issues.


Your recommendation did in fact solve the problem. I don’t get any imaginary frequencies when I freeze core, then optimize, then calculate properties.

Thank you very much for taking the time to reply. I appreciate it!