Tuesday, December 11, 2012

Basis Set Recommendations for DFT Calculations of Gas-Phase Optical Rotation at Different Wavelengths

Hedegård, E. D.; Jensen, F.; Kongsted, J. J. Chem. Theory Comput. 2012, 8, 4425
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission

What is the appropriate basis set to use for computing optical rotations? Hedgård, Jensen, and Kongsted examined the optical rotation of 1-6 using B3LYP and CAM-B3LYP at two different wavelengths.1 They examined a series of different basis sets, including the aug-pCS sets2 (developed for NMR computations), the aug-cc-pVXZ series and 6-311++G(3df,3pd). They compared the computed optical rotation with the different basis sets with the value obtained from an extrapolated basis set computation. The mean absolute deviation using either B3LYP or CAM-B3LYP at the two different basis sets are listed in Table 1. The bottom line is that aug-pcS-2 is the preferred method, but this basis set is rather large and computations of big molecules will be difficult. The aug-pcS-1 set is the best choice for large molecules. Errors with the extensive Pople basis set and the aug-cc-pVXZ sets are quite sizable and of concern (especially at the shorter wavelength). It should also be mentioned that even with the largest aug-pcS basis sets extrapolated to the CBS limit, the computed value of the optical rotation of 3 has the wrong sign! Clearly, basis set choice is not the only issue of concern. We remain in need of a robust methodology for computing optical activity.
Table 1. Mean absolute deviation of the optical activities of 1-6 evaluated at two wavelengths.

589.3 nm
355.0 nm
Basis set
B3LYP
CAM-B3LYP
B3LYP
CAM-B3LYP
aug-pcS-1
4.5
2.2
20.8
15.3
aug-pcS-2
1.4
1.1
4.0
1.5
aug-cc-pVDZ
15.6
13.6
62.2
144.1
aug-cc-pVTZ
3.9
6.3
9.2
37.0
6-311++G(3df,3pd)
6.4
10.3
20.5
40.7

References

(1) Hedegård, E. D.; Jensen, F.; Kongsted, J. "Basis Set Recommendations for DFT Calculations of Gas-Phase Optical Rotation at Different Wavelengths," J. Chem. Theory Comput. 20128, 4425-4433, DOI:10.1021/ct300359s
(2) Jensen, F. "Basis Set Convergence of Nuclear Magnetic Shielding Constants Calculated by Density Functional Methods," J. Chem. Theory Comput. 20084, 719-727, DOI: 10.1021/ct800013z


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