What is the correct way to calculate the BSSE correction for a molecule having 3 fragments? Please see the attached image for details. I know the values are pretty close but still I would love to know which is the correct way.
Dear Soumya Ranjan Dash, If you have three fragments and want to calculate the interaction energy between all three, the correct way to apply the counterpoise correction is to set counterpoise=3. As you know, BSSE is the error generated by the superposition of the basis set. If you take two fragments as one (counterpoise=2), then you introduce some errors yourself by not properly describing the actual system.
Setting counterpoise=3 ensures that the correction is applied to all three fragments individually, preventing errors in calculating the interaction energy.
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