I intend to optimize AB, A, and B separately at a dispersion-corrected level/basis set. Determine the stabilities of the optimized structures through a frequency calculation. Perform single-point energy calculations using the verified (optimized) structures; where E(int) = E(AB) – E(A) - E(B). Please what I’m I missing? Also you’ve got this business of BSSE (?) . In a case where the optimized AB complex has a non-zero imaginary frequency how does one "fix" that?
Dear Bright,
The selection of the level depends on the number of atoms each compound contains. You can run B3LYP/6-31G(d,p) or B3LYP/6-311+G(d,p). The second one is a higher level than the first one.
Now you have three structures A, B and A-B. You have to run optimization of each of the three using the frequency keyword. An example of line 2 in the input file:
# B3LYP/6-311+G(d,p)Opt=(Z-matrix,MaxCycle=300,CalcFC) GFINPUT IOP(6/7=3)
and found the global minimum (GM) structure for each. The search for the GM can be done by conducting rotation around a bond or changing dihedral angle in increment depending on the nature of the structure.
A, B and AB are ground state geometries, therefore, the frequencies calculations should give NO negative value.
Please note, if you have only one negative value the structure will be transition state and if the geometry has more than one negative frequency value the structure will be imaginary.
In your case, A, B and AB should have only positive frequencies.
In case one of them or more have negative frequency it means that the structure is incorrect and should be reoptimize after making changes to the initial geometry.
The output file for each structure gives enthalpy and entropy. Using these two terms for the three structures you will find the interaction energy.
Please note that delta G = delta H -Tdelta S
Where G is the free energy, H is enthalpy, S is entropy and T is a temperature in Kelvin.
Hoping this will be helpful,
Rafik
The best explanation of BSSE and CP correction can be accessed at: http://vergil.chemistry.gatech.edu/notes/cp.pdf
Dear Bright,
Please take into account that if you are after the interaction energy, A and B NEED TO BE evaluated at the complex (AB) geometry. That is, do not use E(A)opt and E(B)opt in E(int) = E(AB) – E(A) - E(B). Instead, use E(A) and E(B) obtained from the AB geometry.
To account for BSSE in G09, you need to add "Fragment=1" and "Fragment=2" for A and B following the corresponding atom labels. Also, add "counterpoise=2" in the input line. You have to specify the charges and multiplicities for every species (that is, A, B, and AB) For instance:
# method/basis counterpoise=2
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Title Card Required
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0 1 0 1 0 1
N(Fragment=1) -9.21985900 -4.94069200 -0.38635000
C(Fragment=1) -8.86528900 -3.59212200 0.10531000
...
...
...
C(Fragment=2) -7.65603900 -3.07557200 -0.66993000
O(Fragment=2) -6.64670900 -3.78783200 -0.84884000
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Dear Bright,
If you intent to have interaction energy, you do not need to optimize A and B separately, having AB dimer optimized structure is enough. Once you have dimer optimized structure you can calculate interaction energy via counterpoise keyword in G09.
To understand the BSSE, I refer you to "Intermolecular interaction" written by Kaplan page 124-129.
In addition to understand the difference(s) between interaction energy, binding energy and dissociation energy I suggest "Ideas of quantum chemistry" written by Piela, page 684-687.
Best wishes,
Sirous
I have a query about BSSE dimer .
Is it possible for BSSE to have negative value?
Best Regards
Himanshu Goel
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