Dear everyone,
I have some molecular systems that I optimized (at BP86/def2-SVP) and then performed a single-point calculation (at B3-LYP/def2-TZVP) on the optimized geometries. When I compared the relative energies between different systems, I got reasonable numbers, lets say 10 - 100 kcal/mol. Now when I treat the same systems with COSMO (at B3-LYP/def2-TZVP and eps=4, 20, 50....etc) and then compare the 'same' relative energies, the results are a bit too large, such as the relative energy value which was 20 kcal/mol (at B3-LYP/def2-TZVP) becomes almost 2400 kcal/mol (at B3-LYP/def2-TZVP and eps=4). I have used Turbomole before, but always before relative energies varied only a little when COSMO treatment was performed.
I can see in output (in COSMO run) that during each SCF iteration energy remains indeed close to what it was in a 'non' COSMO treatment, but a great drop in total energy occurs after ENERGY CONVERGED statement in the output file, but that could be because of the switch of the grid size to a more finer one.
Could you please suggest possible reasons for such large changes in relative energies upon COSMO treatment ? Is it possible that the two above points are linked ? I think I may 'not' have made a mistake somewhere.
many thanks,
best regards,
Vivek