I'm performing VPT2 calculations in a reaction mechanism to calculate rate coefficients by SCTST theory and perceived that some species presented unexpected negative frequencies after the VPT2 calculation. This was observed for transition structures and reactive intermediates bound by hydrogen bonds or other weak interactions. The example below shows my problem:
Fundamental Bands
-----------------
Mode(n) Status E(harm) E(anharm) Aa(x) Ba(y) Ca(z)
1(1) active 3763.110 3598.689 3.057697 0.123142 0.122807
H 2(1) active 2586.244 2577.498 3.060523 0.122682 0.122379
H 3(1) active 2585.777 2554.772 3.063117 0.122701 0.122411
4(1) active 2557.751 2524.031 3.054464 0.122649 0.122352
5(1) active 2418.710 2039.750 3.069775 0.120913 0.120608
6(1) active 1158.445 1122.073 3.141484 0.122739 0.122411
7(1) active 1150.289 1085.902 3.123672 0.122768 0.122499
8(1) active 1008.896 966.784 3.033002 0.122639 0.122347
H 9(1) active 1007.937 958.193 3.030136 0.122625 0.122394
10(1) active 992.893 899.923 3.068002 0.122841 0.122468
11(1) active 305.153 200.472 3.874636 0.122220 0.121213
12(1) active 277.188 187.185 2.355992 0.122516 0.123005
13(1) active 159.127 -393.075 3.079803 0.118279 0.117974
14(1) active 139.159 -112.063 3.112331 0.124439 0.124194
15(1) active 136.961 -99.655 3.098757 0.124517 0.124199
It is possible to see that the 13-14 modes are negative after the VPT2 procedure, while the harmonic frequencies are all positive. I have read that if the perturbation magnitude is higher than the harmonic frequency, and presents a negative value, it can result in a negative frequency. However, I don't know how to improve the calculation to avoid this error.
I'm will be very grateful if anyone has a clue about this issue or helps me to correct my calculations.
In a transition state structure, the frequency corresponding to the reaction coordinate is negative. This is the characteristic feature of the TS structure. it will not go away if your TS is correct.
For other negative frequencies in TS structure or other structures, you can simply remove them by adding displacement manually in the result manu--> vibrations --> manual displacement. Then reoptimize structures and recalculate vibretional frequencies. After a few trials, unwanted negative frequencies will be gone.
@Keshav I think you miss his point slightly. The harmonic frequencies are positive, therefore we are at a minimum.
It is true that the VPT2 method can give negative frequencies. This happens as the anharmonic correction is far too large. In other words - our 0th order guess of a harmonic oscillator is crap for some modes. It is especially the low frequency-modes that are very sensitive to this issue.
You can select which modes to treat anharmonically by 'selectanharmonic' keyword. By not choosing the modes that are troublesome they are only treated harmonically and therefore should prevent negative frequencies.
Bear in mind that the 1 or 2 modes that turns out negative might not be the problem. Maybe it's actually another mode coupling to these that are the problem so you have to make some guestimates of which modes to include and which modes to leave out of the VPT2 treatment.
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