Imaginary frequencies with k-Point calculations

[Karthik]

Hi, 
I am trying to calculate the binding energies of multiple adsorbates on a 
tin oxide surface (a 4 layer slab), as well as the zero point energies. 
When I relax the geometry of the structure (only the top two layers and 
adsorbates are allowed to relax) using a gamma point calculation and run a 
hessian analysis on the optimized structure, all calculated frequencies are 
real. But, when I run a geometry relaxation with k-Points on the optimized 
structure from a gamma point calculation and run a subsequent hessian 
calculation also using k-Points, multiple imaginary frequencies are 
observed. I have verified that the geometries obtained from both gamma 
point and k-Point calculation are almost the same. Can anyone help me with 
what is going wrong here ? 

All input and output files are attached. 

ads_gamma_rlxn.inp/.out (gamma point geometry relaxation) 
ads_gamma_hessian.inp/.out (gamma point hessian analysis)

ads_kP_rlxn.inp/.out (k-point geometry relaxation) 
ads_kP_hessian.inp/.out (k-point hessian analysis)

Thanks for your help,

- Karthik 



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