I am trying to calculate adsorption energies of acetophenone on a 6x6x3 nickel surface with about 12 Angstrom of vacuum above it using SIESTA 5.
I have arranged the molecule so that the ring plane is parallel to the surface plane, and the atom of the molecule closest to the top is about 3.5 Angstrom away.
I have been running several tests, varying parameters and geometric constraints, always keeping the nickel fixed, but I always obtain results that do not seem energetically reasonable to me because I notice too much energy difference between steps.
The energy values I observe for the first steps are
The values according to the output are in eV units. Since these are weak interactions, the energy changes between the different steps seem exaggerated to me.
I have inspected the intermediate geometries and they seems to be reasonable, it is hard to tell the difference by simple inspection. The moving distances are reasonably small and I started the process from an optimized acetophenone.
The data that I believe are most relevant from the simulation are
Mesh.Cutoff 175 Ry
%block LatticeVectors #In Angstroms
14.906518 0.0 0.0
7.4532590 12.909423 0.0
0.0 0.0 30.0855605
Given the size of the SYSTEM (see the lattice vectors), I understand that using only 1 k-point should give at least reasonable results. Is this correct?
To choose the Mesh.Cutoff I analyzed the convergence both when calculating the surface and the molecule, and I was moderately generous. The value is quite larger than the one suggested in the pseudopotentials.
What I am missing here?
Thanks in advance