I curiously have done some anharmonic vibrational calculations using Gaussian (#p freq=(anharmonic,noraman,savenormalmodes) b3lyp/genecp scf=(maxcyc=1000,tight) scrf=cpcm) and the results puzzled me. There are some scattered negative energies as follows:

Mode(n) E(harm) E(anharm) I(anharm)

23(2) 501.552 -2944.908 1.34638814

24(2) 379.580 287.716 0.06309016

25(2) 231.178 214.829 0.13180786

26(2) 196.350 -572.731 0.16901805

27(2) 147.571 -1020.501 2.74354452

28(2) 130.415 130.325 0.28258578

29(2) 58.015 -236.539 0.01753439

30(2) 48.885 103.598 0.01357292

Does anyone know what this means and how to resolve those for a better interpretation of the effect of anharmonicity on this system? Could it be related to the implicit solvent?