What is the correct procedure to calculate solvation model?
1) first optimize the structure in gas phase then again optimize the optimized structure for solvation model? or
2) Directly optimize the structure by solvation model?
In solvation model, do we need to optimize the H2(gas) and H2O(298K = liquid) too if we are taking H2O as the solvent?
There is nothing like "correct" and "incorrect" procedures. There are two alternative approaches, however the final result sometimes may differ. Optimization in gas phase and reoptimization with solvent effect is generally just refining of the stationary/transition state. "Direct" optimization with solvent effect of initial structure should normall end up with the same state. This is very probable if the initial geometry is relatively close to final state, however if it is far from the final state, the optimization with solvent may sometimes proceed in a different direction. Personally I can say that optimization with solvent effect is often more problematic regarding convergence, so i would recommend to perform gas phase optimization and reoptimization with solvent.
Mr. Verma,
PCM is incompatible with minimize energy computations. So that if you have included ...Opt SCRF=PCM Pop=(Minimal) Test, meaning point 2 in your posting, than you should receive and DG of a structure, however there is not know is it corresponds to this one at the potential energy minimum. To evaluate the difference of the molecular structure between gas - phase and solution, than you should compute the contribution of the thermal motion of the atoms at a set of temperature values and than to evaluate the effect on DG via PCM, using solvents with different polarity.
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It is true that an independent solvent calculation may end in a different minimum that if the starting geometry corresponds to the optimized structure in vacuum. However I think that knowing about the reactivity of your system will help you to choose not only the solvent but also the solvent model (needless to say as well about previous theoretical or experimental related work).
Yes, this is absolutely right. There are several approaches of solvent model - PCM, CPCM, COSMO etc.
Please tell me the route for changing the temperature in thermochemistry
'route'???
keyword for changing temperature would be simply
temperature=A
where A should be in Kelvin.
Hi Mr. Shukla
keyword for temp effect would be
# opt freq b3lyp/6-31g(d) geom=connectivity temperature=350 test
Temperature should be in K. Level of theory you can change as you want.
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