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Some Problems (Precautions) of ab initio Quantum Chemistry

Some Problems (Precautions) of ab initio Quantum Chemistry. Jim Jr-Min Lin. An actual situation:. Most papers only show the “good” sides of the calculation and hide the problematic sides. Such that, experimentalists tend to think that theory is nearly perfect. ab initio calculations. e -.

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Some Problems (Precautions) of ab initio Quantum Chemistry

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  1. Some Problems (Precautions) of ab initio Quantum Chemistry Jim Jr-Min Lin An actual situation: Most papers only show the “good” sides of the calculation and hide the problematic sides. Such that, experimentalists tend to think that theory is nearly perfect.

  2. ab initio calculations e- Orbitals 1s, 2s, 2p, ... e- Configurations (1s)2(2s)2(2p)1... (1s)2(2s)1(2p)2 Wave Function Correlation Energy

  3. Methods: (Approximation, Algorithm) HF-SCF CAS-SCF CAS-PT2 (Rayleigh-Schrodiger) MP2 (Moller-Plesset) CCSD(T)/QCISD(T) Density Functional B3-LYP B3-Half and Half

  4. Basis Set (BS): numerical functions to fit orbitals Gaussian BS 6-31G(d,p) 6-311++G(3df,3p) Dunning’s correlation consistent BS cc-pVTZ, cc-pVQZ, cc-pV5Z: cc-pVNZ, N=3,4,5 aug-cc-pVNZ, N=3,4,5 Number of Zeta, z

  5. One electron Wave Function => Approximate Orbital more number of z, more diffuse (s, p, d, f, ... ) l=(1,2, 3, 4, ...) Polarization Functions

  6. JPC, A 101, 7923 (1997)

  7. Even methods that are widely used and known of high accuracy may have Significant Errors. F2 Energy Hartree R(F-F)

  8. JCP 127, 164317 (2007)

  9. Usually big basis set can’t solve the Defect of a method F2

  10. Basis set size has significant effect on accuracy of relative energy and less effect on the geometry. F2

  11. Another Example of Basis Set size ClF

  12. Another Example of Methods ClF

  13. The ‘Shape’ of Potential Energy Curve depends on the Methods F2

  14. Complete Basis Set Extrapolation ClF cc-pVNZ, N=3,4,5 => N=infinity De(Cl-F) Hartree v3z v∞z 0.01H=6.3 kcal/mol v5z v4z (N+0.5)-4

  15. Take Home Messages • Error of ab initio calculation depends on geometry {Ri}and bonding character. Err = f({Ri}) • Form of f ({Ri}) : very sensitive to Methods, • not so sensitive to Basis Set • Especially for Transition State, different methods often predict quite different geometry. • Most methods behave well at equilibrium geometries (Here the bonding is well described by most MO methods) • Testing and Calibration are necessary. • Most papers only show the good sides.

  16. Check your results with other methods. Check if a Multi-Reference (MR) calculation is necessary.

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