MOLEBSSE
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I am sorry for the ignorant question:
what is the purpose of MOLEBSSE keyword (I mean I know what is the purpose, I am confused about its usability)? I am running a classical CRYSTAL and MOLECULE calculation (extracted from CRYSTAL at crystalline geometry) and the same MOLECULE extracted from crystal and optimized to calculate the formation energy (E(crystal)-n*E(molecule)) and strain energy (E(molecule in crystalline geometry)-E(optimized molecule)). For the fully optimized molecule (to get lattice or cohesive energy) I extract the unit with the keyword MOLECULE the molecule that composes the crystal, and optimize it. No MOLEBSSE needed here.
BUt then I would also like to see this "strain" energy, e.g. how much that molecule reorganizes from the crystalline geometry during optimization in a gas phase. But for that, I can again do single point with MOLECULE extraction from a crystal, compare that with fully optimized molecule and I am done. MOLECULE keyword generates a single molecule in gas phase. There are no neighboring molecules (that would contribute their basis sets which can be corrected with BSSE) when one uses MOLECULE keyword. So why have MOLEBSSE here? I knew why but I somehow lost it when I was thinking about it, sorry
Jonas
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Hi Jonas,
As you correctly point out, in this case the molecular calculations are not affected by BSSE as you are extracting a single molecule (i.e. with no neighbors).
The calculation that is affected by BSSE is the one on the crystal, where each molecule within the molecular crystal could potentially "benefit" from the basis functions of the neighboring molecules if its own basis is not complete. So we use MOLEBSSE for this, to make sure we compare apples with apples (i.e. the energy of a molecule in the crystal that has access to the basis functions from neighboring molecules, with the energy of an isolated molecule that also has access to the same neighboring basis functions).
Hope this helps clarifying things a little,
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I am sorry, Alessandro, in what case an isolated molecule have access to neighboring functions? If it is isolated, it has no neighbors and we are back to square one and this is why I wrote this post. Can you give an exampe?
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Of course, in no case an isolated molecule would have access to neighboring functions. But in the crystal it does. If its own basis is not complete, a molecule in the crystal can get extra-stabilized by the neighboring functions not because of a "physical" or "chemical" interaction with neighboring molecules but because of an artifact due to the incompleteness of its basis. So if you were to compute the cohesive energy just as energy of the crystal - energy of the bare molecule, you would get an extra unphysical interaction. To make sure you get the right cohesive energy you could do one of two things: i) remove the extra interaction from the calculation on the crystal; ii) add it also to the molecular calculation so that it cancels when taking the difference (this is the MOLEBSSE strategy, also known as Boys-Bernardi counterpoise correction).
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I sort of understand. Let me look again at the famous urea cohesive energy tutorial and get back if I still need help
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So the right cohesive energy is Ecrystal-Emolecule(BSSE at crystal geometry)?
The "strain" energy is Emolecule(noBSSE at crystal geometry)-Emolecule(gas optimized) -
By strain I mean conformational energy. I found old Mimo presentation on urea I'm studying to refresh the procedure
