I'm attempting to optimize n-alkane dimers using MP2/DFT in Gaussian09. However the first one or two frequencies are always negative. Can these modes be ignored or do I need to do a tighter optimization?
Summarizing the OP's comment that describes how they managed to remove negative (imaginary) frequencies for their system:
- Ensure correct symmetry of the molecular geometry
- Include D3 dispersion for the DFT calculation
- Use an ultrafine integration grid for the SCF
- Use the DFT-D3 Hessians as a guess to perform the MP2 optimization/Freq.